in Metaphysics, that which constitutes the particular nature of each genus or kind, and distinguishes it from all others: being nothing but that abstract idea to which this name is affixed, so that everything contained in it is essential to that particular kind.

This Mr Locke calls the nominal essence; in contradistinction to the real essence, or constitution of substances on which this nominal essence depends. Thus the nominal essence of gold is that complex idea the word gold stands for; let it be, for instance, a body, yellow, weighty, malleable, fusible, and fixed: but its real essence is the constitution of its infensible parts, on which these qualities and all its other properties depend, which is wholly unknown to us.