A telefrag is a special kind of frag or kill that results when a player or monster stands on the location of a teleporter as another player teleports to that location. The monster or player who was originally standing at that location will instantly die. Telefrags can be strategically used by a Doom player to kill powerful monsters without using any ammo (one example is on level E4M2: Perfect Hatred, a player can enter a secret teleport to telefrag the Cyberdemon). Monsters are usually not permitted to teleport to a space that is occupied, preventing them from telefragging. The only situation in which they are permited to telefrag a player is when they try to teleport in MAP30 of any WAD, because it is needed by the spawn shooter cubes to spawn a new monster, so it will not get stuck inside any other (some source ports allow the player to remove this exception or apply it to all maps). A player who is invulnerable or using the God mode cheat will not be protected from telefrags. A barrel that is placed on a teleporter destination can be telefragged and will explode as if it were shot, with the incoming player teleporting into the center of the explosion.
Invulnerability or "invincibility" only protects a player from attacks doing less than 1000 points of damage, whereas a telefrag inflicts 10000 points (see the function P_DamageMobj in p_inter.c). The Doom source code refers to a telefrag as a "stomp".