[RiverTheNuts]

A donk bet is leading into an aggressor when you arent planning to 3 bet or make some move later in the hand to extract further value.
Leading a flopped 4 flush into a big field = good idea
Betting into TAG w/ 2 pair on a rag board, calling a raise, and then c/ring the turn = good idea
Leading into PFR with 73o on an AKQ board = donk bet

Most 2+2ers think a donk bet is made OOP on the turn or river in order to 3 bet it, or to get aggressor to call a bet when he would have otherwise checked behind, such as rivering a one card flush, or picking up 2 pair on the river on a scary board for a PFR