In linguistics, grammatical genders, also called noun classes, are classes of nouns reflected in the behavior of associated words; every noun must belong to one of the classes and there should be very few which belong to several classes at once. (Source of definition: Hockett, 1958, p. 231. See References section.) In other words, in a hypothetical example language that has grammatical genders masculine and feminine, all nouns belong to one of those two genders; in order to use the correct rule of declension for any noun and any modifier (demonstrative, adjective, article, etc.) affecting that noun, one must know whether that noun is feminine or masculine.
The term grammatical gender is mostly used for Indo-European languages, many of which follow the pattern just described.
Grammatical gender is distinguished from natural gender by the fact that grammatical gender requires agreement between nouns and the forms of modifiers (demonstratives, articles, adjectives, etc.), and sometimes even verbs, used in a sentence, whereas natural gender does not.Air Zoom Vomero 14