Using bouncing or swinging momentum to force a joint past its normal range, as opposed to holding a position or moving through it under control.
The classic example is bouncing at the bottom of a toe touch. Momentum takes the tissue past where the client can control it, which is what makes it different from dynamic stretching, where the movement is controlled through a range the client owns.
It is not appropriate for general clients. The stretch reflex fires against the bounce, the end range is not controlled, and the same range gains are available from controlled methods without the risk.
Where it has a place is late-stage athletic preparation, in athletes whose sport contains ballistic end-range positions anyway, such as hurdlers and martial artists, and only after a full warm-up. For everyone else, use dynamic movements before training and static holds after.