Full Idea
If we specify the 'necessary' conditions that are 'sufficient' for something's being an X, that is a combination of conditions such that all and only Xs meet them, which is the hallmark of a definition of X-hood.
Gist of Idea
A definition of a thing gives all the requirements which add up to a guarantee of it
Source
Stephen Davies (The Philosophy of Art (2nd ed) [2016], 2.1)
Book Reference
Davies,Stephen: 'The Philosophy of Art (2nd ed)' [Wiley Blackwell 2016], p.24
A Reaction
There are, of course, many other ways to define something, as shown in the 2.D Reason | Definition section of this database. This nicely summarises the classical view.
Related Idea
Idea 20388 'Necessary' conditions are requirements, and 'sufficient' conditions are guarantees [Davies,S]