Self-destruct

Self-destruct (also known as SD) is when a player kills himself. Most often, a SD is involuntary, and almost always implies player's failure, like a bad-timing recover move or to step on a mine. Sometimes is a game glitch which causes the player to SD, but this is very rare, and the experienced players know where are the main glitches and avoid them (or use them in advantage).

Also, a self-destruct can be voluntary, like in time match mode, when a player with high damage self-destructs for reborn at 0% damage and avoid any other rival to KO him; but this is considered a dirty strategy. Melee (and presumably Brawl too) provides a way (in advanced options menu) to discount more points for players who SD.