[1] : The Collaborative International Dictionary of English v.0.48

torsion \tor"sion\, n. f., fr. ll. torsio, fr. l. torquere,
   tortum, to twist. see torture.
   1. the act of turning or twisting, or the state of being
      twisted; the twisting or wrenching of a body by the
      exertion of a lateral force tending to turn one end or
      part of it about a longitudinal axis, while the other is
      held fast or turned in the opposite direction.
      1913 webster

   2. mech. that force with which a thread, wire, or rod of
      any material, returns, or tends to return, to a state of
      rest after it has been twisted; torsibility.
      1913 webster

   angle of torsion of a curve geom., the indefinitely
      small angle between two consecutive osculating planes of a
      curve of double curvature.

   moment of torsion mech. the moment of a pair of equal and
      opposite couples which tend to twist a body.

   torsion balance physics., an instrument for estimating
      very minute forces, as electric or magnetic attractions
      and repulsions, by the torsion of a very slender wire or
      fiber having at its lower extremity a horizontal bar or
      needle, upon which the forces act.

   torsion scale, a scale for weighing in which the fulcra of
      the levers or beams are strained wires or strips acting by
      torsion.
      1913 webster
see also:
torture angle of torsion moment of torsion torsion balance torsion scale 

[2] : WordNet (r) 2.0

torsion balance
     n : measuring instrument designed to measure small forces by the
         torsion they exert on a thin wire

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