What is "chorale prelude"

n. a composition for organ using a chorale as a basis for variations

In music, a chorale prelude is a short liturgical composition for organ using a chorale tune as its basis. It was a predominant style of the German Baroque era and reached its culmination in the works of J.S. Bach, who wrote 46 (with a 47th unfinished) examples of the form in his Orgelbüchlein.