a delivery designed to go under the bat and hit the stumps
A full-pitched delivery that is aimed at the batsman's toes and/or the base of the stumps. If the ball is swinging, these can be the most lethal delivery in the game, as perfected by Waqar Younis in his pomp
A ball bowled so as to bounce on the pitch on or near the batsman's popping crease.
In cricket, a Yorker is a delivery where the cricket ball bounces on the cricket pitch on or near the batsman's popping crease. Since a batsman in a normal stance has his feet on the popping crease, this means the ball is bouncing very near his feet, which makes the ball difficult to hit with the bat.