an adherent or supporter of a person, group, party, or cause, esp. a person who shows a biased, emotional allegiance.

Military . a member of a party of light or irregular troops engaged in harassing an enemy, esp. a member of a guerrilla band engaged in fighting or sabotage against an occupying army.

of, pertaining to, or characteristic of partisans; partial to a specific party, person, etc.

partisan politics.

of, pertaining to, or carried on by military partisans or guerrillas.

noun

one-sidedness , partiality , prejudice , prepossession , tendentiousness