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]

WordNet (r) 3.0 (2006):

torsion balance
    n 1: measuring instrument designed to measure small forces by
         the torsion they exert on a thin wire