A stick that exceeds the height, width, length, weight and curve restrictions of an allowed hockey stick, or that is of a different material than a usual hockey stick. Illegal sticks are not uncommon in the NHL, however, a player caught using an illegal stick will receive a minor penalty.
The NHL has strict restrictions on the dimensions (height, width, and length), the maximum weight and the maximum curve allowed on a hockey stick. The most noticeable modification is in terms of the curve on a hockey stick. Curving the stick allows the puck to elevate easier, thus allowing the players to have more effective snapshots and wrist shots. Due to this, many sticks become illegal because of the extended curve they have.
If a player notices that one of his opponents is playing with a stick that appears to be illegal, the player must notify the referee of this while the illegal stick is being used on the ice. The referee will then go and inspect the stick and will sometimes ask for a measurement of the stick to be taken. If the stick is found to be illegal, the player that was using it will receive a minor penalty. However, if the stick turns out to be legal, the other team will receive a delay-of-game penalty.
The most famous illegal stick situation in the NHL occurred near the end of game two of the 1993 Stanley Cup finals between the Los Angeles Kings and the Montreal Canadiens. With the Canadiens trailing the game late in the third period and desperately needing a goal, they notified the referee to check the stick of Kings defenceman Marty McSorley, which appeared to be illegal. After inspection, the stick was indeed illegal, and the Kings were given a minor penalty. On the ensuing power play, the Canadiens scored, tying up a game that they eventually won in overtime.