Spawning is the creation of a thing in a level, particularly during play. In the Doom games in-game significant spawning is present as respawning (where things return to the game), from attacks that spawn missiles, and through the final boss that generates monsters. Other item creations, like dropped items, blood splats and other effects can also be called spawning. Other games based on the Doom engine or source ports may provide additional forms of in-game spawning, such as with scripting (Hexen's ACS, for example).
When the respawn setting is applied, any monsters that leave corpses behind when defeated and that initially start off in a level (this thus excludes those generated by other things or monsters) which are felled during play respawn at their original starting point, this also applies if the monster is gibbed or if the monster is squished into a pool of blood by a door or crushing ceiling.
A monster will not spawn if something is occupying its starting location, it may still spawn later if the location is freed, except on MAP30 of any WAD of the Doom II based games (e.g Final Doom) or any other customized wads that use the Doom II level sequence; where the monster would still respawn even if their starting location is occupied where they would telefrag any living organism occupying their starting location upon respawning.
The respawning happens after a randomized time delay (roughly 8 seconds to 5 minutes). When a monster respawns its corpse is whisked away with a teleport flash while another teleport flash marks the arrival of its new incarnation or replacement. Respawned monsters that had been targeting the player before death instantly become alert and move towards their target, but those who had been targeting other monsters or players that are dead are spawned in a nonactive state. The respawn setting can be applied with any skill level using the -respawn parameter in the command line, but it will always be used when the "Nightmare!" skill level is chosen.
In version 1.5 of Doom a v2.0 deathmatch mode was added that allows certain items to respawn after a period in a way similar to monsters respawning (see above), applying a bluish teleportation flash with a unique teleportation noise where the item is respawning. For purposes of game balance, the partial invisibility and invulnerability powerups do not respawn. Items may respawn even if some thing has moved onto their location, and thus a player may stand at the location where an item will return to get it quickly upon its arrival. This setting is applied when the -altdeath parameter is added.
After being killed, any player in a multiplayer game will respawn once a key or button assigned to key_use is pressed by the user controlling that player. During cooperative games the player will respawn at that player's starting location, and during deathmatch games, at a random deathmatch start point. Like with monsters, returning players will arrive in the level with a teleport flash, but player corpses are not removed when the player returns, giving a rough visual cue during play of how many times each player has died.