What is the definition of definition?

Solution 1:

A definition asserts the meaning of a word. Preferably, it asserts a meaning clearly enough that you can distinguish when that word would and would not apply.

"An apple is a fruit" could be called a very poor definition if you wanted to stretch the term.

"An apple is the round fruit of a tree belonging to the rose family" is better but doesn't reliably distinguish between apples and pears.

"Apple (noun): (1) the pomaceous fruit of the apple tree, Malus domestica, of the rose family. (2) the apple tree" has enough detail to be useful.

Solution 2:

One analysis of different types of definition given for lexemes / polysemes by lexicographers is presented by Howard Jackson in Lexicography: An Introduction.

One principle put forward is that 'different forms of definition are appropriate to different types of word [eg aardvark; to] [Zgusta, 1971].

Another is that 'apt wording' should be found to construct a 'telling definition'.

An example of a 'good' definition given is the one from NODE:

'a solid-hoofed plant-eating domesticated mammal with a flowing mane and tail, used for riding, racing, and to carry and pull loads'

You can work out the word being defined.

But with 'a fruit'?