Definitional fallacies

Definitional fallacies are logical fallacies that occur when some definition fails to properly explain some term.

Definitional fallacies are fallacies of ambiguity and informal fallacies.

Circularity
If a term is defined by itself, defining the term as such adds nothing useful.

Too broad
The fallacy of "too broad", or "discarded differentia", occurs if when defining a term, instances are included that aren't usually called by that term.

For example:
 * A fish is an aquatic vertebrate.

This definition would include animals like whales and turtles, which are not fish, so the definition needs to be refined:
 * A fish is a cold-blooded aquatic vertebrate with two sets of paired fins that is covered with scales.

This definition now excludes those animals that wouldn't be considered fish.

Too Narrow
The fallacy of "too narrow" occurs if when defining a term, instances aren't included that are usually called by that term.

For example:
 * A rectangle is an object with four perpendicular sides of equal length.

This would exclude all rectangles that aren't square. Instead, rectangle must be defined as:


 * A rectangle is an object with four sides, each one perpendicular to its adjoining sides.

Ambiguity
A term is ambiguous if it's unclear what it means.