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 consigneeSee UCC § 9-102(a)(28).

See Creditor (contrast).