The notion of grammatical category has a great number of different definitions. And still in all of them the key words are general and obligatory. Keeping that in mind we should develop such definition of this notion. Which corresponds our understanding Grammar as grammar is a system in language which presents meaning through opposition of variants of units. A definition which meets these requirements was proposed by A.I. Snirnitsky more than fifty years ago. This definition might be worded as follows: A grammatical category is a general meaning realised through formal and meaningful opposition of variant of one and the same unit and obligatory expressed by all units of the class[2]. It means that a category consists of at least two forms. These forms of a unit opposed to each other we shall call categorial forms. A categorial form may not coincide with a grammatical form. If a unit has several grammatical categories a grammatical form may be a member of several oppositions and represent several category forms. For example the grammatical form "woman's" represents two category forms — Possessive Case (opposed to the Common Case "woman") and Singular (opposed to Plural "women’s").
This definition makes it possible to describe specific features by which we can recognise it. A.I. Snirnitsky describes them in the following way:
1) Every grammatical category must be represented not less than by two forms. It is impossible to find, say, category of case having only one case form. The grammatical meaning is oppositive and cannot exists without its counterpart.
2) Not a single categorial form can be found in all forms of a unit, because it must be opposed at least to one other category form of the same unit which other form should also be represented by the forms of this unit.
3) One grammatical form can represent several category forms belonging to different categories. This was mentioned before. Here we can take another example: the sentence "It was a nice day" unites two category forms — Declarative (opposed to Interrogative: "Was the day nice?") and Neutral (opposed to Exclamatory: "Nice was the day!").
4) No grammatical form can represent two (or more) categorial forms simultaneously. In case we find a grammatical form which represent two (or more) categorial forms simultaneously, it means that our definition of the category system is not correct we have to revise it.
5) Every grammatical form of a unit represents at least one category form. If we recognise a variant of a unit as a separate form we have to find a category a category form of which it represents.
6) Besides we should remember that a grammatical category show one meaning more than there are members of the opposition constituting it. If we take the category of Number of Nouns we have nearing of Singular, of Plural and of Number, the latter is incorporated into each use of the form whether it is Singular or Plural.