Full Idea
A 'real definition' (as opposed to a linguistic one) is a statement which gives the essential properties of the things to which a given concept applies.
Gist of Idea
'Real' definitions give the essential properties of things under a concept
Source
Thomas Mautner (Penguin Dictionary of Philosophy [1996], 'definition')
Book Reference
Mautner,Thomas: 'Dictionary of Philosophy' [Penguin 1997], p.126
A Reaction
This is often seen as old-fashioned, Aristotelian, and impossible to achieve, but I like it and aspire to it. One can hardly be precise about which properties are 'essential' to something, but there are clear cases. Your 'gold' had better not be brass.