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 § 9-102(a)(28).
See Creditor (contrast).