vox·el ˈväksəl/ noun
noun: voxel; plural noun: voxels
(in computer-based modeling or graphic simulation) each of an array of elements of volume that constitute a notional three-dimensional space, especially each of an array of discrete elements into which a representation of a three-dimensional object is divided.
A voxel represents a value on a regular grid in three-dimensional space. Voxel is a portmanteau for "volume" and "pixel" where pixel is a combination of "picture" and "element".
Read further and see illustrations here: http://en.wikipedia.org/wiki/Voxel