The Collaborative International Dictionary of English v.0.48:

Retrocession \Re`tro*ces"sion\, n. [Cf. F. r['e]trocession. See
   Retrocede.]
   1. The act of retroceding.
      [1913 Webster]

   2. The state of being retroceded, or granted back.
      [1913 Webster]

   3. (Med.) Metastasis of an eruption or a tumor from the
      surface to the interior of the body.
      [1913 Webster]

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

RETROCESSION, civil law. When the assignee of heritable rights conveys his
rights back to the cedent, it is called a retrocession. Erskine, Prin. B. 3,
t. 5, n. 1; Dict. do Jur. h.t.