# Definitional fallacies

 Part of the series on Key articles General logic Bad logic v - t - e
Not to be confused with No True Scotsman -- when one redefines a term mid-debate.

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

The fallacies are fallacies of ambiguity and informal fallacies.

## Circularity

See the main article on this topic: Tautology

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

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 usually not considered fish, so needs to be refined:

A fish is a cold-blooded aquatic vertebrate with two sets of paired fins and 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

See the main article on this topic: Fallacy of ambiguity

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