A self-destruct (abbreviated as SD) is when a character is KO'd without being hit by an opponent. This usually equates as jumping off the edge, but falls also count as SDs when the stage KOs someone. 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 Super Smash Bros. Melee and Super Smash Bros. 4. In bonus mode, the player's score is lowered by 500 points. If characters get defeated by their own item, it counts as both a fall and an SD, making them lose an additional point (Item Self-Destruct worth -1000). Friendly Fire also accounts for a loss of additional points. The character with the most self-destructs gets the Master of Disaster bonus (-1000), unless they have double or more the SDs of whoever has the second-most (and at least 3), in which case they get Self-Destructor instead (-2000).
Defeating a teammate counts as a self-destruct. In Super Smash Bros. however, the character that defeated the ally earns a point while the ally loses a point, similar to a regular KO.
In Super Smash Bros. Brawl and Super Smash Bros. 4, however, a SD can only be attained if the KO'd character did not get hit by an opponent during the entire stock or for three entire minutes, as touching the ground after being hit does not reset their KO property. Therefore, a character can be hit by any attack (or even be Footstool Jumped off of), return to the stage, continue to play for a while, and KO themselves, but it will not count as an SD. Players, however, will often refer to any fall that is clearly the player's own fault as a self-destruct, regardless of whether the game counts it as such.
In Super Smash Bros. 4 characters can be credited for a KO when the opponent self-destructs in a timed 1v1 match.