(Ice) hockey:
A player assigned to follow an opposing player whenever the latter is on the ice, in order to prevent him or her from handling the puck, making plays, or otherwise contributing to the offense.
Being an effective shadow requires quickness and the ability to out-think your opponent. Usually a team will assign a checking forward to shadow the opponents' superstar player. A bit of a pugnacious attitude helps, too! If the shadow can annoy his or her target into taking a foolish penalty, so much the better - the target is off the ice, and the shadow's team gets a power play!