Arrears (or arrearage) is a legal term for the part of a debt that is overdue after missing one or more required payments. The amount of the arrears is the amount accrued from the date on which the first missed payment was due. The term is usually used in relation with periodically recurring payments such as rent, bills, royalties (or other contractual payments), and child support.
Payment in arrear is a payment made after a service has been provided, as distinct from in advance, which are payments made at the start of a period. For instance, rent is usually paid in advance, but mortgages in arrears...