Webster's English Dictionary

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relativity

rel.a.tiv.i.ty \.rel-*-'tiv-*t-e-\ n 1: the quality or state of being 
   relative  2: the state of being dependent for existence on or determined in 
   nature, v alue, or quality by relation to something else 3a: a theory 
   formulated by Albert Einstein and leading to the assertion of  the 
   equivalence of mass and energy and of the increase of the mass of a body 
   with increased velocity and based on the two postulates that if two systems 
   are in relative motion with uniform linear velocity it is impossible for 
   observers in either system by observation and measurement of phenomena in 
   the other to learn more about the motion than the fact that it is relative 
   motion and that measurements of the velocity of light in either system 
   regardless of the position of the source of light always give the same 
   numerical value 3b: an extension of the theory to include a discussion of 
   gravitation and r elated phenomena 4: RELATIVISM