Equivocation

Marriage is a fine institution, but I'm not ready for an institution. Equivocation is a logical fallacy that relies on implicitly alternating between the different meanings a single word can have in different contexts.

When the switch is purposefully made obvious, the result is often humorous (see above quote). The term for this is a. When the switch is done as quietly as possible, however, the result becomes misleading instead.

It is a favorite fallacy of creationist and woo hucksters alike. It can be used both to seemingly belittle a concept or idea, or to raise one up to false pretenses. It can be used to disguise bullshit such as in quantum woo where people like Deepak Chopra use equivocation to make their ideas sound like they have some basis in reality.

Equivocation is an informal fallacy and a fallacy of ambiguity, although its parent fallacy, the four-term fallacy, is a formal fallacy.

Form
With letters representing words, this may seem obviously false, but when words are introduced, the fallacy becomes much less apparent.

Obviously false
In this case, the word "is" in the two premises is used differently in meaning than "is equivalent to" in the conclusion.

Less obvious

 * "Because it's called the theory of evolution, evolution is an unproven assumption!"
 * This is fallacious because it equivocates two different meanings of the word "theory."


 * "Because the New World Order is a conspiracy theory, it is fact.
 * This is fallacious because it equivocates two different meanings of the word "theory," but in the inverted case.


 * "You have faith in science, and I have faith in God."
 * This exploits the fact that "faith" is often used to mean "confidence", that is having a solid reason to be confident of something due to its past performance — this argument is an effort to equate science with religion, when in reality science does not require faith (meaning belief without evidence). Although assuming the word faith always implies "blind faith" is an extremely common example of this fallacy.


 * "The theory of relativity is immoral because it promotes moral relativism."
 * This equivocates two different meanings of the word "relative."