A definition is a statement of the meaning of a term (a word, phrase, or other set of symbols).[1][2] Definitions can be classified into two large categories, intensional definitions (which try to give the sense of a term) and extensional definitions (which try to list the objects that a term describes).[3] Another important category of definitions is the class of ostensive definitions, which convey the meaning of a term by pointing out examples. A term may have many different senses and multiple meanings, and thus require multiple definitions.[4][a]

In mathematics, a definition is used to give a precise meaning to a new term, by describing a condition which unambiguously qualifies what a mathematical term is and is not.[5] Definitions and axioms form the basis on which all of modern mathematics is to be constructed.[6]