I really wish the nhl rulebook was more publicly available. But some googling found these two examples:
Rule 42 of the NHL rulebook dictates that charging "shall mean the actions of a player or goalkeeper who, as a result of distance traveled, shall violently check an opponent in any manner. A 'charge' may be the result of a check into the boards, into the goal frame or in open ice."
Official NHL Charging Rule Text (Rule 607)
“Charging is the action where a player takes more than two strides or travels an excessive distance to accelerate through a body check for the purpose of punishing the opponent. This includes skating or leaving one’s feet (jumping) into the opponent to deliver a check, accelerating through a check for the purpose of punishing the opponent, or skating a great distance for the purpose of delivering a check with excessive force.”