Basically, physics operates at a wide range of scales from microscopic to cosmic, so the classical definitions have to be extended to make sense at every level. Mass is different from weight. See http://en.wikipedia.org/wiki/Mass_versus_weight There are various concepts of mass : 1) rest mass 2) relativistic mass. 3) invariant mass See http://en.wikipedia.org/wiki/Mass%E2%80%93energy_equivalence