A shot taken from a "set", or stationary, position. The shooter releases the shot without jumping.
Set shooters are rare in today's NBA because if a player doesn't jump when he releases the ball, it makes it easy for a defender to block the shot. For that reason, set shooters are usually taller players, such as centers and power forwards.
However, almost all players perform a set shot when they are shooting a free throw, since there's no defense to block the shot.