Full Idea
Things being ranked under Names into sorts only as they agree with certain abstract ideas, to which we have annexed the Names, the essence of each sort comes to nothing but that abstract idea which the sortal name stands for. This is the nominal Essence.
Gist of Idea
The nominal essence is the idea behind a name used for sorting
Source
David Wiggins (Sameness and Substance Renewed [2001], III.iii.15)
Book Reference
Locke,John: 'Essay Concerning Human Understanding', ed/tr. Nidditch,P.H. [OUP 1979], p.417
A Reaction
He contrasts 'nominal essence' with 'real essence'. A key passage for David Wiggins. One shouldn't put too much emphasis on nominal essence, since it means that someone referred to as 'that idiot over there' (you, perhaps) is necessarily an idiot.