An action of skating after the offense, angling, using one's stick, bodychecking or disrupting the puck carrier in anyway to force a turnover or take away a scoring chance. It can also be used to describe a defensive player who is assigned to shut down an offensive member of the other team. ie. Tonight, he is checking Wayne Gretzky.
Checking is often thought of when bodychecks are delivered. However, many other types of checking are done throughout a game - forechecking, backchecking, poke-checking and stick checking for example.
In 2005 the National Hockey League - NHL clamped down on rules to make it more difficult to check opponents. Prior to making the rules more strict, checking including being able to hook, hold and sometimes slash opponents as long as it wasn't dangerous or caused a turnover. Now players must do more stick checking and position themselves to take away scoring chances.