Pump and Dump is a scheme where a group artificially inflates the price of a low-liquidity coin (pump) and then sells at the top (dump), leaving late buyers with heavy losses.
Example:
A Telegram group hypes a little-known token and pushes its price from $1 to $4 in hours. Insiders then dump their holdings, and the price crashes back under $1, while new buyers get stuck holding the bag.