Donk bet
A donk bet is a lead bet into the player who was the aggressor on the previous street, instead of checking to let them bet first, and it is uncommon outside boards that clearly favour the caller's range.
A donk bet flips the usual order of action. Normally the player who called a bet or raise checks to the previous aggressor and waits to see whether they bet again. A donk bet skips that step: the caller leads out first, without waiting to be checked to, on the very next street.
Solvers reach for this rarely, because leading usually gives up information and lets the aggressor raise with the read that you decided to bet rather than trap. It tends to make sense only when the new card shifts the range advantage sharply enough that betting first outperforms checking, typically a card that connects far better with the range the preflop caller defended than with the range the raiser attacked with.
Worked example. You call a preflop raise in the big blind and the flop comes 8h7h6s, a low, connected board that hits the calling range's suited connectors and small pairs much harder than the raiser's unpaired big cards. Leading out here for 8bb into a 20bb pot, rather than checking to the raiser, can charge a range that mostly missed before it gets a free look at the turn.