A microburst is an intense, but short-lived, downdraft.
Microbursts are a significant concern during low-altitude phases of flight. The downdraft experienced can be as much as 6,000 feet per minute.
Reference(s):
FAA AC 00-6B Aviation Weather pg. 19-6
FAA-H-8083-25B Pilot’s Handbook of Aeronautical Knowledge 12-11