Full Idea
Essential definitions are such as are taken from the essential differences of things, which constitute them in such a sort of natural bodies, and discriminate them from all those of any other sort.
Gist of Idea
Essential definitions show the differences that discriminate things, and make them what they are
Source
Robert Boyle (The Origin of Forms and Qualities [1666], p.41?), quoted by Peter Alexander - Ideas, Qualities and Corpuscles
Book Reference
Alexander,Peter: 'Ideas, Qualities and Corpuscles' [CUP 1985], p.56
A Reaction
I don't think this goes as far as the aim Aristotle had in definitions, which was more than merely to 'discriminate' each thing. A full definition explains the thing as well.