What is Debtor?

Legal Definition
A debtor is an entity that owes a debt to another entity. The entity may be an individual, a firm, a government, a company or other legal person. The counterparty is called a creditor. When the counterpart of this debt arrangement is a bank, the debtor is more often referred to as a borrower.

If X borrowed money from his/her bank, X is the debtor and the bank is the creditor. If X puts money in the bank, X is the creditor and the bank is the debtor.

It is not a crime to fail to pay a debt. Except in certain bankruptcy situations, debtors can choose to pay debts in any priority they choose. But if one fails to pay a debt, they have broken a contract or agreement between them and a creditor. Generally, most oral and written agreements for the repayment of consumer debt - debts for personal, family or household purposes secured primarily by a person's residence - are enforceable.

However, for the most part, debts that are business related must be made in writing to be enforceable by law. If the written agreement requires the debtor to pay a specific amount of money, then the creditor does not have to accept any lesser amount, and should be paid in full.

Also, if there was no actual agreement but the creditor has proven to have loaned an amount of money, undertaken services or given the debtor a product, the debtor must then pay the creditor.
-- Wikipedia
Legal Definition
In general, someone who owes an obligation to someone else. Most commonly, an obligation to pay money. In specific contexts:

1. In bankruptcy law, someone who files a voluntary bankruptcy petition, or against whom an involuntary bankruptcy petition gets filed.

2. In secured transactions, any of the following: (1) someone with a property interest in collateral, (2) a seller of accounts, chattel paper, payment intangibles, or promissory notes, and (3) a consignee. See UCC ยง 9-102(a)(28).

See Creditor (contrast).
Legal Definition
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 generall 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.
-- Bouviers Law Dictionary
Legal Definition
One who owes a debt; he who may be compelled to pay a claim or demand.
See also
-- Black's Law Dictionary
Legal Definition
One who owes a debt.
-- Ballentine's Law Dictionary