A grammatical category is a set of syntactic features that:
The term ‘grammatical category’ has been used to cover a wide variety of things, including what traditional grammars call "parts of speech."
Hopper, Paul J. 1992. "Grammaticalization." In Bright 1992
Crystal, David. 1985.A dictionary of linguistics and phonetics. 2nd edition. New York: Basil Blackwell.
Bybee, Joan L. 1985.Morphology: A study of the relation between meaning and form. Amsterdam: John Benjamins.