Covert category

A covert category is a semantic distinction that is relevant for the morphosyntactic rules of a language, but is not expressed overtly by lexical or morphological means.

Examples
C]overt categories in English are e.g. mass vs. count noun, animacy, stative vs. dynamic verbs.


 * "The second assumption I wish to make explicit is the importance of covert categories. Many recent and not-so-recent studies have convinced us of the relevance of grammatical properties lacking obvious ‘morphemic’ realizations but having a reality that can be observed on the basis of selectional constraints and transformational possibilities. We are constantly finding that grammatical features found in one language show up in some form or other in other languages as well, if we have the subtlety it takes to discover covert categories." (Fillmore 1968:3)

Origin
The term was apparently coined by Benjamin Lee Whorf.

Reference

 * Fillmore, Charles J. 1968. The case for case. In: Bach, Emmon & Harms, R.T. (eds.) Universals in linguistic theory. New York: Holt, Rinehart & Winston, 1-88.