crusader

Concise Oxford English Dictionary © 2008 Oxford University Press:
crusade /kruːˈseɪd/

▶noun

• 1 any of a series of medieval military expeditions made by Europeans to recover the Holy Land from the Muslims.

  ■ a war instigated for alleged religious ends.

• 2 a vigorous campaign with a political, social, or religious aim: a crusade against crime.

▶verb (often as adj. crusading) lead or take part in a crusade.

– derivatives
crusader noun.

– origin C16: from Fr. croisade, alt. (influenced by Sp. cruzado) of earlier croisée, lit. ‘the state of being marked with the cross’, based on L. crux, cruc- ‘cross’.