A definition is used to make clear the concept that a term refers to. A definition normally has a set of conditions; if something fits all the conditions, then it is an instance of the term, otherwise it is not.

The Fallacies of Definition are a class of logical fallacies in which the definition doesn't make clear the concept. For example, a definition of square should let someone, without help, identify every square in existence. If that person misses some squares, or includes items that are not squares, or cannot determine if something is or is not a square, then the definition was not clear.