A 3D pixel. Contraction of Volume Element. Imagine aproximating a 3D object to a 3D mesh, and you'll get the idea. If pixels are squares, then voxels are cubes; though people tend to draw them as rectangles to save memory and time. Used in demos and video / computer games (eg, Commanche Maximum Overkill) to display nice landscapes, and more recently (in Tiberian Sun and Blade Runner) for characters and objects. Some people have said that voxels are the future, since they could represent a truely changable environment, but due to the massive memory requirements, and the fact that they cannot be accelerated adequitely, this is a long way off. Voxels are used in the scientific community to represent samples taken in a 3D space in the same way 2D data can be represented as an image.

Addendum: Err... I've just been told that pixels are not squares, and voxels are not cubes... But you get the idea. They are samples taken at a periodic distance. The sample is used to represent a volume with the dimension of that sample distance around the point sampled. Also, there was a story on /. recently (Aug 2000) about a company making high end voxel accelerators, so maybe the time of voxels is closer than we think.