Telefrag

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 exceptions to this are spawn shooter cubes, which telefrag anything on the spot they land on, and all monsters in MAP30 of any WAD (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 can still be killed by a telefrag.

Utilizing an intentional telefrag is considered cheating in a UV pacifist speedrun.

Technical
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".