Grammatical category is a unity of grammatical form and grammatical meaning

Grammar is a system of rules which governs the production and use of utterances in a given language. Grammar has its fundamental notions. The basic notions dealing with the analysis of the categorical structure of the word: grammatical form, grammatical meaning, grammatical category, paradigm. A grammatical category is a class of units (such as noun and verb) or features (such as number and case) that share a common set of grammatical properties. Also called grammatical feature.R.L. Trask notes that usage of the term category in linguistics "is so varied that no general definition is possible; in practice, a category is simply any class of related grammatical objects which someone wants to consider" (A Dictionary of Grammatical Terms in Linguistics, 1996).