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The Collaborative International Dictionary of English v.0.48 (gcide)

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]