Source : Webster's Revised Unabridged Dictionary (1913)

Relativity \Rel`a*tiv"i*ty\ (-t?v"?-t?), n.
   The state of being relative; as, the relativity of a subject.
   --Coleridge.

Source : WordNet®

relativity
     n : (physics) the theory that space and time are relative
         concepts rather than absolute concepts [syn: {theory of
         relativity}, {relativity theory}, {Einstein's theory of
         relativity}]