Definition is the description of the meaning of a word, phrase, or concept. There are various kinds of definition, including truth-functional, recursive (or inductive), lexical, stipulative, ostensive, definition by abstraction, and definition by genus and difference. The truth-functional definitions and definitions by abstraction are the most commonly used in logic and mathematics.

A lexical definition is the report of a meaning a word already has. This is mainly what a dictionary definition is. A stipulative definition is one which assigns a new meaning to a word. Stipulative definitions are often used by theoreticians attempting to use a word in a way that will capture an idea in a way not thought previously. An ostensive definition is an example of the word to be defined. Pointing to an apple would be an ostensive definition of the word, "apple." The definiendum (that which is defined in a definition) in a...