Voxel
A 
voxel is a "volumetric pixel". Voxel models are in many ways essentially 3D sprites, allowing them to fit visually much better with the rest of a Doom-engine game's graphics than polygonal models would.
ZDoom supports voxels using the KVX format employed in Build games Blood and Shadow Warrior. The SLADE3 editor is able to identify voxel lumps in the VOX and KVX format. Voxels can be given any name, however, it is recommended to use the same naming convention as sprites, with the exception that rotation angles are irrelevant and therefore omitted, and mirroring is not supported. Additional informations can be provided with the VOXELDEF lump.
The 
Doom voxel project currently exists under the form of a pack of voxel model replacements for various Doom actors, as well as experiments with voxelized walls and switches using dummy actors.
Voxels can be created in both VOX and KVX formats, as well as KV6, with Ken Silverman's SLAB6 editor. Voxels must be allocated within a wad file between VX_START and VX_END markers, meanwhile within a .pk3 archive they may simply be placed within a "voxels" folder.
Voxel display can be disabled by setting the r_drawvoxels to false.
Note: Voxels support translucency, but not translations.
