A bottomless pit, as its name implies, is a pit that has no identifiable bottom; a common hazard in many video games.
Depending on the game, when a player character falls into a bottomless pit, they may lose either a life or health and/or be forced to restart from the last checkpoint, often regardless of invincibility or any other power-ups the player may have obtained. In some games, this can be avoided through a special item. e.g., in Mega Man 7, the robotic bird, Beat, can prevent Mega Man from losing a life.
In first-person shooters, pits are generally not bottomless and fall damage may instead result in damage or death. Other alternatives to the bottomless pit include water, lava, spikes, quicksand, and crushing, all of which may vary in damage.