I've seen several good definitions here for Pump and Dump.

Here's my own definition from https://decryptionary.com/dictionary/pump-and-dump/

An illegal manipulation tactic where people increase the price of a stock (the pump) so that they can sell it at those high prices for a profit (the dump). The person creating the pump and dump must first own some of the asset. Then they convince others to buy the asset with exaggerated or fake news causing the price to rise. At that point, they sell what they own. With no one promoting the asset, prices drop and many people are left with worthless assets and wasted money.

"Pump and dump" is often said in the crypto space to mean sharing news in forums (like this) and chat groups like Telegram to encourage people to buy a crypto. Then as more people begin buying, the price rises and sellers can get rid of their coins to these new buyers at a profit. Because the SEC is not yet regulating the crypto space, it is not illegal.