8-bit

8-bit is a collective term that refers to an address space that is 8 bits wide.

Graphics
The first release of Doom, as well as Doom II used 256 color graphics which used an 8-bit palette though there were thousands of shades for color available. Other 8-bit graphics include an 8-bit alpha channel that puts translucency in modern source ports of Doom in whith it makes a total 32-bit graphics mode when combined with 24-bit truecolor space.

Sound
The first release of Doom and its derivatives had some 8-bit quality audio modes available for the soundtrack.

Miscellaneous
Megaman 8-bit Deathmatch is a version of Skulltag that uses 8-bit style retro theming, though the engine itself is 32-bit.

No proper version of Doom used an 8-bit processor as its main chip.