A voxel refers to a single datapoint in a 3d-grid, in the same way a Pixel refers to a datapoint on a 2d-grid. The word is a combination of "volume" and "pixel".
The volume of a voxel and the data stored can be anything.
For example, the Terrain system in Roblox allow creators to fill a grid of 3x3x3 voxels with different material each with different occupancy values (ie how full of the material they are).
This Roblox Script creates a single voxel of water with 100% occupancy (used as the cover image).