A pleat is a creased fold in fabric. Your long wool coat might have a single pleat in back, down toward the hem.
Some pleats are creased sharply, or even stitched firmly in place, while others are looser. The pleats in a little girl's frilly dress or in a skirt that has pleats all the way around might be called "accordion pleats," while a kilt usually has one "box pleat" in the back, with simple "knife pleats" on either side. Pleat is also a verb, meaning "to make or sew a pleat."