One of a set of numbers defined in terms of an invariant generated by the finite cyclic covering spaces of a Knot complement. The torsion numbers for Knots up to 9 crossings were cataloged by Reidemeister (1948).
References
Reidemeister, K. Knotentheorie. New York: Chelsea, 1948.
Rolfsen, D. ``Torsion Numbers.'' §6A in Knots and Links. Wilmington, DE:
Publish or Perish Press, pp. 145-146, 1976.