To kill another being(s) by teleporting into the space they are occupying.

Many first-person shooter games have teleporters. Telefragging was afaik invented in DOOM, where if you teleport into space already occupied by a monster or player, you take over that space, fragging the monster or player.
In the final map of Quake, the way to kill the boss is to enter a teleporter when the other end of that teleporter enters the boss, thereby telefragging it.

To frag through teleportation. First appearance in the game of Doom.

To kill an opponent in a game of Quake by respawning or teleporting directly on top/into them.

using the translocator to kill an opponent in Unreal Tournamnet

In Halo, to attempt to continue a teleport while another player is standing on the tele pad, despite warnings of "The Teleporter is Blocked"
The player on the teleport is then killed, dubbed a 'Telefrag.'
Taken from Unreal Tournament.

In the game Medal of Honor, the ability to drop a grenade at a teammate's feet, then switch teams, whereby the grenade now becomes hostile, so when it blows up, it kills your teammate.