Full Idea
The nominal essence of a natural kind K consists of those features a thing must have to deserve the name 'a K' by virtue of the meaning of that name.
Gist of Idea
Nominal essence of a natural kind is the features that make it fit its name
Source
Alexander Bird (Philosophy of Science [1998], Ch.3)
Book Reference
Bird,Alexander: 'Philosophy of Science' [UCL Press 2000], p.99
A Reaction
Some people think 'nominal essence' is the only essence there is, which would make it relative to human languages. The rival view is that there are 'real essences'. I favour the latter view.