In philosophy, the specification of the meaning of an expression relative to a language.
Definitions may be classified as lexical, ostensive, and stipulative. Lexical definition specifies the meaning of an expression by stating it in terms of other expressions whose meaning is assumed to be known (e.g., a ewe is a female sheep). Ostensive definition specifies the meaning of an expression by pointing to examples of things to which the expression applies (e.g., green is the color of grass, limes, lily pads, and emeralds). Stipulative definition assigns a new meaning to an expression (or a meaning to a new expression); the expression defined (definiendum) may either be a new expression that is being introduced into the language for the first time, or an expression that is already current.