A collective noun is 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:
Swan (Practical English Usage, New Edition, Oxford University Press, 1997) elaborates on this singular/plural usage, and disagrees about treating collective nouns as both singular and plural in the same construction:
“In British English, singular words like family, team, government, which refer to groups of people, can be used with either singular or plural verbs and pronouns.
This team is/are going to lose.
Plural forms are common when the group is considered as a collection of people doing personal things like deciding, hoping or wanting; and in these cases we use who, not which, as a relative pronoun. Singular forms (with which as a relative pronoun) are more common when the group is seen as an impersonal unit. Compare:
My family have decided to move to Nottingham. They think it’s a better place to live.
The average British family has 3.6 members. It is smaller and richer than 50 years ago.
The government, who are hoping to ease export restrictions soon, …
The government, which is elected by a simple majority, …
My firm are wonderful. They do all they can for me.
My firm was founded in the 18th century.
When a group noun is used with a singular determiner (e.g. a/an, each, every, this, that), singular verbs and pronouns are normal. Compare:
The team are full of enthusiasm.
A team which is full of enthusiasm has a better chance of winning.
Sometimes singular and plural forms are mixed:
The group gave its first concert in June and they are already booked up for the next six months.
Examples of group nouns which can be used with both singular and plural verbs in British English:
England (e.g. the football team)
In American English singular verbs are normally used with most of these nouns in all cases (though family can have a plural verb). Plural pronouns can be used:
The team is in Detroit this weekend. They have a good chance of winning.”