1. An unpaid or an overdue debt or an unfulfilled obligation; usually used in the plural form.
2. A situation when someone is behind in fulfilling obligations: "His car payments are in arrears; and as a result, his car may be repossessed."
3. Money that is not paid when due usually, the sum of a series of unpaid amounts; such as, rent, installments on an account, a promissory note, or monthly child support.

Sometimes these are called "arrearages" or the state or condition of being in arrears.

4. Arrearages are overdue alimony or child support payments.

Child support arrearages can't be discharged in bankruptcy, and courts usually will not retroactively cancel them; so, a spouse or parent who falls on tough times and is unable to make payments should request a temporary modification of the payments before the arrearages build up.

5. Etymology: from Old French ariere, "behind, backward"; from Common (Vulgar) Latin ad retro, from Latin ad, "to" + retro, "behind".