Which term describes an unpaid debt or payment that is overdue?

The correct term that describes an unpaid debt or payment that is overdue is "Arrearage." This term specifically refers to the amount of money that is owed and has not yet been paid by the due date. In legal contexts, it is often used to describe overdue payments related to various obligations, such as child support, alimony, or loans.

In contrast, "Alimony" refers to financial support that one spouse may be required to pay to another after separation or divorce, but it does not inherently mean the payment is overdue. "Custody" pertains to the legal guardianship of children and has no relation to debts. "Support Order" indicates a legal decree mandating the payment of support, such as child support or spousal support, but like alimony, it does not specifically indicate that the payment is overdue. Therefore, "Arrearage" is the precise term for unpaid and overdue debts.