Dump is a terms used to refer to a situation in open markets when an asset plummets in price.
A “dump” in prices can happen for a multitude of reasons; bad news are released, early investors exist their positions, too much FUD is circulating, a pump & dump scheme is executed, the list is endless.
The main difference between a Dump and other forms of market selloffs is that dumps are very abrupt and happen spontaneously.