An example of a punishment. The landing lag produced by Snake's forward aerial gives enough time for King Dedede to connect his dash attack.
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 punishable 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. A move's power can make punishment seem more dangerous; some players may choose to avoid the forward smashes of Ike and King Dedede rather than attempt to punish them simply because of the risk involved.
Some characters can use attacks with super armor for a unique kind of punish: for example, Ganondorf's Warlock Punch in Super Smash Bros. for Nintendo 3DS and Wii U can punish a bad attacking decision (such as aledge attack) due to its small amount of super armor, while Little Mac's smash attacks all provide enough super armor for him to punish any sort of laggy attack, weak or strong, by simply plowing through it.
Characters can also be punished if they have used a move that causes helplessness, or they are in a position where their only recovery option is obvious and predictable.