Nukage

Nukage refers to pools or rivers of radioactive waste, which are damaging to players that step into them (but not monsters). Strictly speaking, nukage is green and slime is brown, but since the effect on the player is usually the same, the terms are often used interchangably.

Despite the damaging effects, it is often necessary to cross nukage during gameplay. Crossing as quickly as possible minimizes the damage. Also, the radiation suit protects the player from damage for a limited time.

Barrels can also be said to contain nukage, since the game booklet says they contain "fuel, toxic waste, or some other volatile substance".

Technical information
The term comes from the names of flats in the WAD file (NUKAGE1, NUKAGE2 and NUKAGE3) that are used as the floor texture for damaging, green pools and rivers. The Doom rendering engine rotates through all three textures to animate the floor and thus make it appear liquid.

In fact, the texture is not what makes a floor damaging: a special sector type is used to control damage. Types are defined that inflict 5%, 10%, and 20% damage per second while the player is within the typed sector. (Damage is halved at skill level 1.) The same sector types are used, but with different flats, to make damaging slime, lava, etc.

If the player has armor, then the damage is split between health and armor.