The Collaborative International Dictionary of English v.0.48:

Retorsion \Re*tor"sion\, n.
   Same as Retortion.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Retortion \Re*tor"tion\, n. [Cf. F. r['e]torsion. See Retort,
   v. t.]
   1. Act of retorting or throwing back; reflection or turning
      back. [Written also retorsion.]
      [1913 Webster]

            It was, however, necessary to possess some single
            term expressive of this intellectual retortion.
                                                  --Sir W.
                                                  Hamilton.
      [1913 Webster]

   2. (Law) Retaliation. --Wharton.
      [1913 Webster]

Bouvier's Law Dictionary, Revised 6th Ed (1856):

RETORSION, war. The name of the act employed by a government to impose the
same hard treatment on the citizens or subjects of a state, that the latter
has used towards the citizens or subjects of the former, for the purpose of
obtaining the removal of obnoxious measures. Vattel, liv. 2, c. 18, Sec.
341; De Martens, Precis, liv. 8, c. 2, Sec. 254; Kluber, Droit dos Gens, s.
2 c. 1, Sec. 234; Mann. Comm. 105.
     2. Retorsion signifies also the act by which an individual returns to
his adversary evil for evil; as, if Peter call Paul thief, and Paul says you
are a greater thief.