Wiktionary
n. A type of freestyle skiing where skiers ski terrain characterized by a large number of different bump called moguls.
Wikipedia
Mogul skiing is a freestyle skiing competition consisting of one timed run of free skiing on a steep, heavily moguled course, stressing technical turns, aerial maneuvers and speed. Internationally, the sport is contested at the FIS Freestyle World Ski Championships, and at the Winter Olympic Games.
Moguls are a series of bumps on a piste formed when skiers push snow into mounds as they do sharp turns. This tends to happen naturally as skiers use the slope but they can also be constructed artificially. Once formed, a naturally occurring mogul tends to grow as skiers follow similar paths around it, further deepening the surrounding grooves known as troughs. Since skiing tends to be a series of linked turns, moguls form together to create a bump field.
The term mogul is from the Bavarian language word "mugel," which means mound or small hill.