Full Idea
That which general words signify is a sort of things; and does it by being a sign of an abstract idea in the mind; ..so that the essences (or species) of things are nothing else but these abstract ideas.
Gist of Idea
A species of thing is an abstract idea, and a word is a sign that refers to the idea
Source
John Locke (Essay Conc Human Understanding (2nd Ed) [1694]), quoted by Stephen P. Schwartz - Intro to Naming,Necessity and Natural Kinds §II
Book Reference
'Naming, Necessity, and Natural Kinds', ed/tr. Schwartz,Stephen P. [Cornell 1979], p.16
A Reaction
This has come in for a lot of criticism, culminating in Putnam saying that meanings 'ain't in the head' (Idea 4099). Wittgenstein's 'beetle in the box' problem is also partly aimed at it (Idea 4147). Locke misses the social aspect of language.
Related Ideas
Idea 4099 If Twins talking about 'water' and 'XYZ' have different thoughts but identical heads, then thoughts aren't in the head [Putnam, by Crane]
Idea 4147 If we only named pain by our own case, it would be like naming beetles by looking in a private box [Wittgenstein]