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.

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.

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.