A voxel is a volume element which represents a value on a three-dimensional grid. In layman's terms, it is a three dimensional pixel. Voxels are frequently used in physical and medical data.
Voxel-based images are frequently used in Blood. Most weapons and power-ups are rendered using voxels, as opposed to sprites, which are two-dimensional images. Many other Build engine games, such as Shadow Warrior used voxels, though Blood was the first.