See collective noun in American Heritage Dictionary 4
noun
A noun that denotes a collection of persons or things regarded as a unit.
Usage Note: In American usage, a collective noun takes a singular verb when it refers to the collection considered as a whole, as in The family was united on this question. The enemy is suing for peace. It takes a plural verb when it refers to the members of the group considered as individuals, as in My family are always fighting among themselves. The enemy were showing up in groups of three or four to turn in their weapons. In British usage, however, collective nouns are more often treated as plurals: The government have not announced a new policy. The team are playing in the test matches next week. A collective noun should not be treated as both singular and plural in the same construction; thus The family is determined to press its (not their) claim. Among the common collective nouns are committee, clergy, company, enemy, group, family, flock, public, and team. See Usage Notes at government, group.