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

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

SUBROGATION, civil law, contracts. The act of putting by a transfer, a
person in the place of another, or a thing in the place of another thing. It
is the substitution (q.v.) of a new for an old creditor, and the succession
to his rights, which is called subrogation; transfusio unius creditoris in
alium. It is precisely the reverse of delegation. (q.v.)
     2. There are three kinds of subrogation: 1. That made by the owner of a
thing of his own free will; example, when be voluntarily assigns it. 2. That
which arises in consequence of the law, even without the consent of the
owner; example, when a man pays a debt which could not be properly called
his own, but which nevertheless it was his interest to pay, or which he
might have been compelled to pay for another, the law subrogates him to all
the rights of the creditor. Vide 2 Binn. Rep. 382; White's L. C. in Eq.* 60-
72. 3. That which arises by the act of law joined to the act of the debtor;
as, when the debtor borrows money expressly to pay off his debt, and with
the intention of substituting the lender in the place of the old creditor. 7
Toull. liv. 3, t. 3, c. 5, sect. 1, Sec. 2. Vide Civ. Code of Louisiana,
art. 2155 to 2158; Merl. Repert. h.t.; Dig. lib. 20; Code, lib. 8, t. 18 et
19 9 Watts. R. 451; 6 Watts & Serg. 190; 2 Bouv. Inst. n. 1413.