Definition

John Stuart Mill, 1868
A proposition declaratory of the meaning of a word; namely, either the meaning which it bears in common acceptation, or that which the speaker or writer … intends to annex to it.”

Mill, J.S., A System of Logic, Ratiocinative and Inductive: Being a Connected View of The Principles of Evidence, and the Methods of Scientific Investigation, 7th ed., Longmans, Green, Reader, and Dyer, London. 1868:149

Wierzbicka, 1996
A definition – in the sense which is relevant to linguistics – is an expression which ‘shows’, so to speak, the meaning of a word by articulating it into its components …by defining a word, then, I mean, essentially what Locke meant: ‘showing’ the meaning of a definable (i.e. semantically complex) word in terms of indefinable (i.e. semantically simple) ones.

Wierzbicka A., Semantics: Primes and Universals. Oxford University Press, Oxford 1996:237