The Collaborative International Dictionary of English v.0.48:

relativity \rel`a*tiv"i*ty\ (-t?v"?-t?), n.
   1. The state of being relative; as, the relativity of a
      subject. --Coleridge.
      [1913 Webster]

   2. One of two theories (also called theory of relativity)
      proposed by Albert Einstein, the special theory of
      relativity, or the general theory of relativity. The
      special theory of relativity or special relativity is
      based on the proposition that the speed of light is a
      constant no matter how observed, and is independent of the
      motion of the observer. From this follows several
      principles, such as the increase of mass with velocity
      (which has been confirmed: see relativistic mass
      equation) and the impossibility of acceleration to a
      speed greater than that of light; the equivalence of mass
      and energy, expressed by the famous equation E = mc^2;
      and time dilation, which is the apparent slowing of a
      clock in a system, as observed by an observer in a system
      moving relative to the clock. The general theory of
      relativity is based on the proposition that there is no
      physical difference between gravitational force and the
      force produced by acceleration. From this follow several
      results, of which the bending of light rays in a
      gravitational field and the equivalence of the inertial
      and gravitational masses have been verified. The possible
      existence of black holes (believed by many astronomers
      to have been adequately proven) is another consequence of
      the theory.
      [PJC]

WordNet (r) 3.0 (2006):

theory of relativity
    n 1: (physics) the theory that space and time are relative
         concepts rather than absolute concepts [syn: relativity,
         theory of relativity, relativity theory, Einstein's
         theory of relativity]