Blast line
From SmashWiki, the Super Smash Bros. wiki
A blast line is a boundary on a stage that a character cannot cross without getting KO'd. Some stages have blast lines that are accessible by simply walking into them, such as the blast lines on either side of either SSB or Melee's Mushroom Kingdom. Most stages, however, contain blast lines on the side that are far away from the ledge, as well as upper and lower blast lines.
The upper blast line is exceptional in that a character can only die by crossing it while on a platform or in hitstun, with the exception of Super Smash Bros. 64, and a fully charged Wario Waft. A character can jump over the upper blast line without dying, as long as he isn't knocked back by an attack. This is the principle behind the Luigi Ladder, in which two Luigis rise higher and higher by trading Super Jump Punches with no knockback.
Non-playable characters such as the Alloys die off the upper blast line as if it were any other blast line (with the exception of the Polygons).
The only way to die is by passing a blast line (excluding Stamina Mode in Melee and Brawl and getting flattened in The Subspace Emissary).
The game's camera refuses to pan or zoom enough during gameplay to actually "see" the blast lines (although the lines themselves are invisible, as can be evidenced when the camera is rotated while pausing). When a character is between a blast line and the camera's view (or above the upper blast line without dying), they will appear in a bubble and slowly take damage. This is known as hoop damage.
In The Subspace Emissary, the blast lines will not KO Player 2 if P2 is not hit by an enemy (similar to the upper blast line in normal gameplay). Player 2 will do an automatic Space Jump instead. Also, the upper blast line does not cause characters to fall into the background or foreground during platforming segments; they're KO'd instantly off the top.
Items and projectiles that cross the blast lines are removed from play (except for, again, the upper blast line). A Smash Ball can freely cross blast lines until it chooses to leave.
