Dumping, as the word is used in crypto markets, is covered in the entry Dump, which explains the term, why dumps happen and how they affect prices. This page gives a short definition and explains the different forms of the word.
Dumping is the rapid sale of a large quantity of a cryptocurrency or token, enough to push its price down sharply. It may be done by a whale taking profits, by early investors or team members whose tokens have just unlocked, or by many holders selling in panic at once. In thin markets, a single dump can overwhelm buy orders and trigger stop-loss orders and liquidations, deepening the fall. Dumping is the second half of a pump-and-dump scheme, in which promoters inflate a price and then sell to the buyers they attracted. Some projects adopt an anti-dumping policy, such as vesting schedules or sale limits, to reduce the risk.
"Dump" is the noun or verb and "dumping" the ongoing activity; both refer to the same thing. The crypto meaning should not be confused with dumping in international trade, which means exporting goods below their fair price.