Argumentum ad dictionarium
“”It depends on what the meaning of the words 'is' is.
|—Bill Clinton - possibly the most egregious example.|
“”Surely if I choose to redefine a spoon as something I shove up my arse, it doesn't stop you enjoying your pudding... unless I used your spoon.
Argumentum ad dictionarium is the act of pulling out a dictionary to support your assertions. More broadly speaking it can refer to any argument about definitions, semantics, or what label to apply to a person or idea - an actual dictionary may not be involved, sometimes the definition is purely personal, sometimes it can be a case of picking and choosing definitions raised by other sources, but the end use is the same. For the most part, "dictionary" is used as a short-cut to refer to any source of these definitions, including statement such as "well, if I define X like this...", which is possibly the most asinine form of the fallacy. See, we've already had to head off one use of this fallacy already in case someone says "it's not this fallacy because I'm not using a dictionary"!
It forms an informal fallacy combining attributes of a red herring argument and, frequently, special pleading. It's very closely related to equivocation. About 90% of arguments on the internet tend to boil down to this.
 But the dictionary said!
|“||Dictionary editors are historians of usage, not legislators of language.||”|
|“||"When I use a word," Humpty Dumpty said, in rather a scornful tone, "it means just what I choose it to mean—neither more nor less."||”|
—Lewis Carroll in Through the Looking-Glass
Because teachers will have inevitably told everyone, at some point in their lives, to look up what words mean in the dictionary, they seem to have taken on a quasi-mythical status as arbitrators in arguments. We're not talking about someone merely using a word that doesn't mean what they think it means, like Vizzini in The Princess Bride, this is about using a definition, or claimed definition, to actually prove a point. In linguistics this would be called "prescriptivism", a school whereby words are prescribed their meaning by an authority - most linguists, philosophers, debaters and anyone sensible, however, are more keen on "descriptivism", whereby dictionaries merely describe how words are used.
However, this is fallacious because a dictionary doesn't actually prove anything. If someone is using a "wrong" word, all they're really doing is using a word in a way that few other people also use. There are good reasons to avoid this, of course, as good and clear communications requires Person A to know what "flibbertygibbit" means when they say it and for Person B to think of (within experimental error) the exact same thing when they hear it. If the two disagree, then the two just disagree on the definition of "flibbertygibbit", and nothing else.
 A quick and dirty thought experiment
Consider the following (intentionally absurd) thought experiment to show how dictionaries don't really prove or change anything in reality. Suppose Alice and Bob are sat down having coffee. After a quick chat over stock prices, they descend into an argument. Alice, annoyed with Bob, decides to play a hilarious prank. Leaving the room she immediately runs to the dictionary on the book shelf. Finding the word "coffee", she scribbles out "beverage made by percolation, infusion, or decoction from the roasted and ground seeds of a coffee plant" and replaces it with "a slimy sticky mixture of soil with a liquid, especially water". Not content that the joke has been played to its full potential, she leaves and runs to the head of Merriam-Webster, then the OED and then finally to Wiktionary itself - and by the powers of magic changes every definition of "coffee" in every dictionary on the planet. Alice then returns the argument and Bob takes another drink.
So, what does Bob taste? A "beverage made by percolation, infusion, or decoction from the roasted and ground seeds of a coffee plant" or some "slimy sticky mixture of soil with water"?
The answer is obviously "beverage from roasted seeds of the coffee plant" and not muddy water, but every argumentum ad dictionarium suggests that somehow the change in definition can somehow affect the reality. It's only because that this example is absurdly literal enough to show it clearly. More subtle examples can mask the fallacy.
 A little background
General or "unabridged" dictionaries (sometimes referred to as "big dics" by people who are mentally eight years old, at least temporarily) are big ol' word lists that give the commonly accepted definitions of words, along with etymologies and contextual notes; for the most part, this is what people usually think of when they say "The Dictionary". These are usually very large books (or, in the case of the Oxford, multivolume sets) and are generally considered the reference of record for word usage in a given language. They're largely divided into prescriptivist (defining the standard register of a language, how it should be used) and descriptivist (how the language actually is used). In English, the most authoritative dictionaries, the Oxford English Dictionary and Webster's International, are strongly descriptivist, and in the case of the recent history of the OED radically so; this is sometimes a source of annoyance to language snobs. This is not necessarily the case in languages with regulating academies; for example, the dictionary of the Académie Française is strongly prescriptivist. (And some dictionaries, like the American Heritage Dictionary, strike a middle ground, following a generally descriptive approach to content while adding usage notes and the like where formal and informal language diverge significantly.)
"Shorter" or "College" dictionaries are abridged, stripped-down versions of general dictionaries, mostly reduced to words and definitions and nothing more. The dictionaries used by computer spellcheckers are even further stripped down, limited only to lists of words and occasionally common grammatical and style errors.
Specialized dictionaries are published for a great many fields of study, especially when (as in law) a field of study has significant numbers of terms of art that differ significantly from standard use. This category also includes etymological dictionaries, which study the history of words and idioms, and slang and dialect dictionaries like the Jargon File or the Dictionary of American Regional English, as well as specialized dictionaries for crossword puzzles, Scrabble, and other wordgames.
Thesauruses (from the Latin word for "treasury") are similar to dictionaries but instead of definitions, provide synonyms, antonyms, and related words.
Bi- and multilingual dictionaries are used to translate words between different languages; usually they don't give actual definitions and can sometimes be thought of as very specialized thesauruses.
Rhyme dictionaries are generally used by poets and songwriters, although they've been found as far back as Classical Chinese.
Finally, humorous dictionaries are a form of literature with a tradition going at least as far back as Samuel Johnson's original dictionary, though the form arguably came into its own with The Devil's Dictionary by Ambrose Bierce. Subsets include dictionaries of humorous neologisms like Rich Hall's Sniglets and Douglas Adams and John Lloyd's Meaning of Liff.
One "argument" against same-sex marriage that has been increasingly prevalent since explicit homophobia became slightly socially taboo, is that marriage has a particular definition, and that same-sex marriage violates that definition. This "definition" claim can be generic, as is the case with the Church of England's stance, or specifically targeted towards child-rearing as is the case with a pamphlet released by The Ruth Institute for Marriage.  The snarky words of PZ Myers, responding to this sort of argument by patent attorney Riley Balling, sums it up nicely:
|I have defined marriage, and marriage is defined this way, and therefore changing the definition of marriage changes marriage by definition. Oh, and my marriage is all about pooping out kids, therefore your marriage damn well better be too.|
Here, the fallacy comes because the definition is ultimately arbitrary - nowhere in the physical universe is such a definition carved out. One can define anything however you like, but facts and opinions are not actually altered by the definition restriction. While other examples given here focus on verifiable physical facts, the argumentum ad dictionarium fallacy equally applies to social constructs like marriage. In fact, the fallacy is highlighted more because definitions are especially arbitrary and subjective in these cases. If one person can "define" marriage as being focused on raising children, that's fine, but someone else - with equal validity - can "define" it as being about securing financial and legal rights for partners.
Feminism suffers from this sort of thing far more than most labels (atheism comes close, but doesn't have the same range of ambiguity). For instance, is feminism simply a pseudonym for "gender equality" or something else? The answer is that it depends on how it is defined. Consider the following prototypical conversation:
- Person A: "Do you think women need to be treated differently?"
- Person B: "No, I think men and women should be equal."
- Person A: "Then you are a feminist, by definition!!"
Further labelling issues from this situation are what E-Prime was invented for, but the main issue is that the label is defined in a certain way, but ignores other connotations that can go with it. This would be fine if all people thought feminism was about was gender equality, but there are people - from Cathy Brennan to Germaine Greer - who hold very unsavoury opinions that deviate from this, despite being considered, and self-identified as, feminists. So, although this dictionary definition might be true and agreed upon by some people, the connections and connotations of the word (that aren't covered by the definition alone) still exist.
Most people consider the problem to be that some people have taken the true definition and distorted it considerably. In reality, the problem is that words only have the power and meaning we give them; merely altering the definition of "feminist" doesn't change this. Simply taking a short-cut straight to what someone believes - whether they believe in gender equality, or whether they think transgender women are "ghastly parodies" of "real" women - gets around having to argue for hours on end on the definition of feminism.
 A few more examples of false precision
The Church of Scientology is somewhat notorious for dictionary abuse; one of L. Ron Hubbard's principles was "never go by a misunderstood word", leading to Scientologists developing what some have called a dictionary fetish. The result is that alleged "misunderstood words" sometimes lead to grueling auditing sessions for no particularly good reason; the Scientology approach is particularly insidious because it ignores idioms.
A slightly different case is sovereign citizen types harping on a specific edition of Black's Law Dictionary; in this case, the dictionary abuser is treating the dictionary's definitions as law, despite the fact that no specific law dictionary is actually considered law in any known jurisdiction, either by legislation or precedent.
And, of course, there's the lazy student school of proofreading, in which as long as a papal passes spiel Czech, it's good to gogh.
An old philosophical problem reads "If a tree falls in the forest and no one is around to hear it, does it make a sound?". While this has multiple interpretations, including a more general "does something really exist if we're not around to experience it?", it's most often taken as a bizarre nonsense argument about the definition of "sound". If Alice says "it does make a sound because sound is waves created by the impact" and Bob says "it doesn't make a sound because sound is only 'sound' when it's processed by ears" both would be right, in a way, but both would most likely interpret it as a disagreement and argue over this definition of "sound" until Alice got annoyed and tried to metaphysically poison Bob's coffee again. The actual facts, that a tree falls, the energy dissipates into the air, the ground shakes, and shockwaves travel outwards, are the same in both cases. Hence, the supposed disagreement, and the apparent dichotomy and paradox of the original question, are illusions.
Even the more general philosophical idea of whether something exists if we don't directly observe it happening does almost the exact same thing with the word "exists". Finding a tree lying in a forest, surrounded by disturbed leaves and broken branches, we can make some reasonable inferences about how it got there, and the fact that it really is lying there can't be readily disputed. Whether or not the event "actually happened" relies not on clever philosophy, but on linguistic trickery over the definition of "actually happened".
Is atheism a belief that God does not exist or a lack of a belief that God exists? This one actually popped up on RationalWiki in a very literal way, but is a reflection of countless other internet arguments. Whatever the "definition", it will not change peoples beliefs and attitudes; changing the definition of "atheist" between "belief in no gods" to "lack of belief in god" won't change how people feel any more than changing it to "one who believes in God" is going to fill the pews in church.
Most commonly, atheism is defined as either of those ideas; the lack of belief, or a belief in no gods. The concepts of strong atheism and weak atheism were developed to accommodate this, although there are good arguments to suggest that when it comes down to figuring out what people believe based around how they act, they're really both exactly the same thing. "Defining" atheism as one or the other would leave a lot of people without a word to describe themselves and nothing more. There is a further extension to this definitionary argument when agnosticism is included. Most traditionally, agnosticism is defined based on one's knowledge - you don't know if God exists - while atheism has been defined based on belief - you don't believe or act as if God is real. But this doesn't stop definition-based arguments being used trying to shunt people from one to the other as if it was a matter of pride. Neil deGrasse Tyson says he prefers "agnostic" over "atheist" purely because of the baggage associated with the latter (though "no label" in preference to both) and even Richard Dawkins has been accused of "going soft" and turning into an agnostic after admitting (even though he spent a good chapter of The God Delusion discussing this) he can only be finitely certain of God's non-existence. However, no matter how you choose to use a label, it doesn't fundamentally alter either of these people's beliefs.
Of course, it gets slightly more interesting when, as with the tree/sound example, the question is not answered but dissolved. A belief in no God and no belief in God are functionally identical. When presented with a hypothetical object that possesses all the qualities associated with "God" and asked the question "do you think this is a real thing?" both strong and weak atheists would answer the same way; no. Even in many cases of "don't know", a person will still act as if the answer is no, and go about their lives as if the answer is no - as is the case with Carl Sagan's fable "The Dragon in My Garage". A more useful distinction than weak/strong is explicit atheism and implicit atheism, where someone's awareness of the question is tested.
 See also
- ↑ It depends...
- ↑ https://www.youtube.com/watch?v=B4g3tQwvjrM
- ↑ See this comment responding to Winston Wu about his "definition" of skeptic.
- ↑ Less Wrong - Disputing Definitions
- ↑ Vizzini: He Didn't fall? Inconceivable!
Inigo Montoya: You keep using that word. I do not think it means what you think it means. 
- ↑ Let's go to Merriam-Webster's. They make good lattes!
- ↑ Old Chinese rhyme dictionaries in particular have proven very valuable to Sino-Tibetan linguists in puzzling out the rather tongue-twisting phonetics of ancient Chinese.
- ↑ [http://www.guardian.co.uk/commentisfree/2012/may/17/justice-equality-same-sex-marriage The Guardian - Redefining marriage to include same-sex couples would benefit nobody, John Sentamu
- ↑ Incidentally modern dictionaries redifine marriage to include same sex marriage. marriage
- ↑ Pharyngula - The same old bad argument against gay marriage
- ↑ Atheism is the belief that God does not exist.
- ↑ Neil deGrasse Tyson - Atheist or Agnostic?
- ↑ Daily Mail - 'I can't be sure God DOES NOT exist': World's most notorious atheist Richard Dawkins admits he is in fact agnostic - the tone of this Daily Mail article should make the point abundantly clear.