posse

Concise Oxford English Dictionary © 2008 Oxford University Press:
posse /ˈpɒsi/

▶noun

• 1 N. Amer. historical a body of men summoned by a sheriff to enforce the law.

• 2 (also posse comitatus /ˌkɒmɪˈteɪtəs/) Brit. historical the body of men above the age of fifteen in a county, whom the sheriff could summon to repress a riot or for other purposes. [comitatus from med. L., ‘of the county’.]

• 3 informal a group of people with a common characteristic, interest, or purpose.

– origin C17: from med. L., lit. ‘power’, from L. posse ‘be able’.