The Collaborative International Dictionary of English v.0.48:

Debtor \Debt"or\, n. [OE. dettur, dettour, OF. detor, detur,
   detour, F. d['e]biteur, fr. L. debitor, fr. debere to owe.
   See Debt.]
   One who owes a debt; one who is indebted; -- correlative to
   creditor.
   [1913 Webster]

         [I 'll] bring your latter hazard back again,
         And thankfully rest debtor for the first. --Shak.
   [1913 Webster]

         In Athens an insolvent debtor became slave to his
         creditor.                                --Mitford.
   [1913 Webster]

         Debtors for our lives to you.            --Tennyson.
   [1913 Webster]

WordNet (r) 3.0 (2006):

debtor
    n 1: a person who owes a creditor; someone who has the
         obligation of paying a debt [syn: debtor, debitor]
         [ant: creditor]

Easton's 1897 Bible Dictionary:

Debtor
   Various regulations as to the relation between debtor and
   creditor are laid down in the Scriptures.

     (1.) The debtor was to deliver up as a pledge to the creditor
   what he could most easily dispense with (Deut. 24:10, 11).

     (2.) A mill, or millstone, or upper garment, when given as a
   pledge, could not be kept over night (Ex. 22:26, 27).

     (3.) A debt could not be exacted during the Sabbatic year
   (Deut. 15:1-15).

     For other laws bearing on this relation see Lev. 25:14, 32,
   39; Matt. 18:25, 34.

     (4.) A surety was liable in the same way as the original
   debtor (Prov. 11:15; 17:18).


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

DEBTOR, contracts. One who owes a debt; he who may be constrained to pay
what he owes.
     2. A debtor is bound to pay his debt personally, and all the estate he
possesses or may acquire, is also liable for his debt.
     3. Debtors are joint or several; joint, when they all equally owe the
debt in solido; in this case if a suit should be necessary to recover the
debt, all the debtors must be sued together or, when some are dead, the
survivors must be sued, but each is bound for the whole debt, having a right
to contribution from the others; they are several, when each promises
severally to pay the whole debt; and obligations are generally binding on
both or all debtors jointly and severally. When they are severally bound
each may be sued separately, and on the payment of debt by one, the others
will be bound to contribution, where all had participated in the money or
property, which was the cause of the debt.
     4. Debtors are also principal and surety; the principal debtor is bound
as between him and his surety to pay the whole debt. and if the surety pay
it, he will be entitled to recover against the principal. Vide Bouv. Inst.
Index, h.t.; Vin. Ab. Creditor and Debtor; Id. Debt; 8 Com. Dig. 288; Dig.
50, 16, 108 Id. 50, 16, 178, 3; Toull. liv. 2, n. 250.