The Collaborative International Dictionary of English v.0.48:

Subrogation \Sub`ro*ga"tion\, n. [Cf. F. subrogation, LL.
   subrogatio.]
   The act of subrogating. Specifically: (Law) The substitution
   of one person in the place of another as a creditor, the new
   creditor succeeding to the rights of the former; the mode by
   which a third person who pays a creditor succeeds to his
   rights against the debtor. --Bouvier. Burrill. Abbott.
   [1913 Webster]

WordNet (r) 3.0 (2006):

subrogation
    n 1: (law) the act of substituting of one creditor for another