NUMBER.
The boy shouts. The boys shout. The form of the subject boy is changed by adding an s to it. The meaning has changed. Boy denotes one lad; boys, two or more lads. This change in the form and the meaning of nouns is called +Number+; the word boy, denoting one thing, is in the +Singular Number+; and boys, denoting more than one thing, is in the +Plural Number+. Number expresses only the distinction of one from more than one; to express more precisely how many, we use adjectives, and say two boys, four boys, many or several boys.