Nukage

Nukage is slang for radioactive nuclear waste. It is most often used in reference to pools or rivers of damaging green fluid. The term comes from the names of animated flats in the IWAD files (NUKAGE1, NUKAGE2 and NUKAGE3).

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

The nukage seen in the early alpha versions of Doom looks slightly different. According to John Romero, this nukage was "too static"; it did not "flow", but just cycled through states.

Source

 * 5 Years of Doom at Doomworld