Corpse

Most enemies leave behind corpses when they die. The exceptions are lost souls and pain elementals. Additionally, arch-viles and nightmare arch-viles leave behind a generic "bloody corpse" tile that doesn't count as a corpse for the purposes of respawning or resurrection. (It does count for various skulls though.)

Gibbing

Gibbing a monster stops it from leaving a corpse and from dropping items. This can be useful if arch-viles are present (or on Nightmare! difficulty).

To gib a monster you must kill it by dealing a large amount of damage. The amount of damage depends on the maximum health of the monster and the damage type of your weapon. Physical and shrapnel weapons must deal 400% of the monster's maximum health to cause a gib. Plasma and acid weapons must deal 200% of maximum health to gib. Fire weapons are the best at gibbing; they only need to deal 150% of maximum health.

It is also possible to gib corpses after the monster has already died. Corpses that are named after enemies (which excludes the arch-vile's "bloody corpse") have 1 HP and as much armor as the corresponding enemy. Corpses don't block bullets, so the only damage that can affect them is from explosions. (In practice, almost every explosion will destroy all corpses in its radius.)

Respawning

On Nightmare! difficulty level, corpses sometimes respawn into new enemies. The newly created enemy is the same kind as the enemy that left the corpse. Respawning is prevented if the corpse is destroyed or if the player or another enemy is standing on the corpse.

Every 10.0s on a level, there is a chance for corpses to respawn. There are two checks for a corpse to respawn: a global check that must be passed if any corpses are to respawn, and then another spawn for each corpse. For the global check, the chance of passing starts at 20% when entering the level and grows by 1% for each subsequent check. The chance of passing the per-corpse checks starts at 10% and increases by 1% for every 100.0s spent on the level.