The Free On-line Dictionary of Computing (30 December 2018):

voxel

    (By analogy with "pixel") Volume element.

   The smallest distinguishable box-shaped part of a
   three-dimensional space.  A particular voxel will be
   identified by the x, y and z coordinates of one of its eight
   corners, or perhaps its centre.  The term is used in three
   dimensional modelling.

   (1995-03-10)