A self-destruct (abbreviated "SD") is the name for a "suicide" in Super Smash Bros. Melee and occurs when a character dies on his or her own without offensive interference by an opponent. (Interference by environment damage, however, such as cars, can still cause a self-destruct). Self-destructs carry various penalties in the different game modes. In Time mode, self-destructs subtract one point by default from a player's score; this penalty can be changed to zero or two points. In Bonus mode, the player's score is lowered by 500 points.
Killing a teammate also counts as a self-destruct.
Self-destructs are more difficult to get in Super Smash Bros. Brawl, as various KOs that would have been a self-destruct in Melee count as regular KOs in Brawl. In Melee an attemped KO will reset to a self-destruct as soon as a character touches ground. However in Brawl if one character were to get hit by another character and fall off the stage even if the first character was able to land beforehand, it would still count towards the second character scoring a KO. In other words, as long as the player doesn't get hurt by the enemy it will count towards a self-destruct.