Punishment
Punishment is the act of attacking an opponent while they are vulnerable, usually from using an attack with slow start-up lag at a non-opportune time or having missed an attack with high ending lag. For example, the Falcon Punch is a punishible move since it has long start-up and ending lag; if Captain Falcon uses it against an active and expecting opponent or if the move's target is able to avoid the attack, they have ample time to launch a counterattack. Characters can also be punished if their attack is shielded, such as via shield grabbing or out of shield attacks. An attack that is difficult to punish is usually known as "safe", usually due to having low ending lag and/or large hitboxes that are difficult to get around.
Characters can also be punished if they have used their up special move and are therefore in helpless, or they are in a position where their only recovery option is obvious and predictable.