1. Liability on a claim; a specific sum of money due by agreement or otherwise <the debt amounted to $2,500>.
2. The aggregate of all existing claims against a person, entity, or state <the bank denied the loan application after analyzing the applicant’s outstanding debt>.
3. A nonmonetary thing that one person owes another, such as goods or services <her debt was to supply him with 10 international first class tickets on the airline of his choice>.
4. A common-law writ by which a court adjudicates claims involving fixed sums of money <he brought suit in debt>. — aka (in the sense of 4) writ of debt. 
Excerpt from Benjamin J. Shipman’s Handbook of Common-Law Pleading:
“The action of debt lies where a party claims the recovery of a debt; that is, a liquidated or certain sum of money due him. The action is based upon contract, but the contract may be implied, either in fact or in law, as well as express; & it may be either a simple contract or a specialty. The most common instances of its use are for debts:
a.) Upon unilateral contracts express or implied in fact.
b.) Upon quasi-contractual obligations having the force & effect of simple contracts.
c.) Upon bonds & covenants under seal.
d.) Upon judgments or obligations of record.
e.) Upon obligations imposed by statute.” 
1. One to whom a debt is owed; one who gives credit for money or goods. — aka debtee.
2. Roman law. One to whom any obligation is owed, whether contractual or otherwise. Cf. DEBITOR.
3. A person or entity with a definite claim against another, especially a claim that is capable of adjustment & liquidation.
4. Bankruptcy. A person or entity having a claim against the debtor predating the order for relief concerning the debtor.
1. To believe <the jury did not credit this testimony>.
2. To enter (as an amount) on the credit side of an account <the account was credited with $500>”
1. Roman law. Someone who has a legal obligation to someone else. Pl. debitores.”
1. Someone who owes an obligation to another, especially an obligation to pay money; especially, the person who owes payment or other performance of a secured obligation, whether or not that person owns or has rights in the collateral — including the seller of accounts, contract rights, or chattel paper.
2. Bankruptcy. A petition or against whom an involuntary petition is filed. — aka bankrupt.
Excerpt from Douglas G. Baird’s Elements of Bankruptcy:
“Section 101 [of the Bankruptcy Code] also introduces us to the language of modern bankruptcy practice. It tells us, for instance, that the person whom a bankruptcy case concerns is a debtor. A person or a firm in bankruptcy is no longer called a bankrupt. Although that word retains some currency among lay people, among bankruptcy lawyers, it sounds old-fashioned & precious.” 
n. Roman law.
1. The act of adapting or conforming to a particular use; orderly regulation or arrangement.
2. That which adapts one thing to another or to a particular use.
3. The act of settling or arranging as a dispute or other difference; SETTLEMENT (3).
4. An amount added or deducted based on settlement.
1. The act of determining by agreement or by litigation the exact amount of something (as a debt or damages) that before was uncertain.
2. The act of settling a debt by payment or other satisfaction.
3. The act or process of converting assets into cash, especially to settle debts.
: Black’s Law Dictionary Deluxe Tenth Edition by Henry Campbell Black & Editor in Chief Bryan A. Garner; page 768. ISBN: 978-0-314-62130-6
: Benjamin J. Shipman, Handbook of Common-Law Pleading § 52, at 132 (Henry Winthrop Ballantine ed., 3d ed. 1923)
: Douglas G. Baird’s Elements of Bankruptcy 6 (2001)