Full Idea
The 'real essence' of a thing is that set of its properties or structures in virtue of which it is a thing of that kind; its 'nominal essence' is the properties or structures in virtue of which it is described as a thing of that kind.
Gist of Idea
'Real essence' makes it what it is; 'nominal essence' makes us categorise it a certain way
Source
Brian Ellis (The Philosophy of Nature: new essentialism [2002], Ch.1)
Book Reference
Ellis,Brian: 'The Philosophy of Nature: new essentialism' [Acumen 2002], p.16
A Reaction
I like this distinction, because it is the kind made by realists like me who are fighting to make philosophers keep their epistemology and their ontology separate.