partisan

Concise Oxford English Dictionary © 2008 Oxford University Press:
partisan /ˈpɑːtɪzan, ˌpɑːtɪˈzan/

▶noun

• 1 a strong supporter of a party, cause, or person.

• 2 a member of an armed group fighting secretly against an occupying force.

▶adjective prejudiced in favour of a particular cause.

– derivatives
partisanship noun.

– origin C16: from Fr., via Ital. dial. from Ital. partigiano, from parte ‘part’.