An illegal block by an offensive or defensive player. In the NFL it is only called on low blocks during kicking plays. In NCAA it is called against any player lined up outside of the tackles that blocks low (unless it is a straight ahead block).
This type of block was banned as a strategy due to the extreme likelihood of knee and lower body injuries. If a player first makes contact above the waist and then lowers down under the waist, it is not a penalty.