In trading slang, a dump is a sudden, heavy wave of selling that drives a price down sharply. The word is most often used for the second stage of a pump and dump scheme, in which promoters push a price up and then sell their holdings to late buyers; that scheme is explained in full on its own page.
A dump can also happen without manipulation, for example when a whale sells a large position or a tokenunlock releases supply. Buyers left holding the asset after a dump are called bagholders.