"military expedition under the banner of the cross," 1706, a respelling or replacement of croisade (1570s), from Middle French croisade (16c.), Spanish cruzada, both from Medieval Latin cruciata, past participle of cruciare "to mark with a cross," from Latin crux (genitive crucis) "cross" (see crux).
The modern English form is comparatively late, and even the earlier croisade is post-Middle English (French croisade replaced earlier croisée). Middle English nouns were croiserie (c. 1300), creiserie.
Especially in reference to the medieval expeditions undertaken by European Christians for recovery of the Holy Land from Muslims. Generally they are counted as seven between 1095 and 1271, but some smaller efforts (e.g. the "Children's Crusade") are omitted and the word sometimes is extended to other religiously motivated expeditions (e.g. against the Albigenses or the Prussians). Figurative sense of "vigorous campaign for a moral cause or against a public evil" is from 1786.