What is Arrears?

Legal Definition
Arrears (also sometimes known as an 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 term describing payments made after a service has been provided.
-- Wikipedia
Legal Definition
A remaining debt or liability after the due date. An example of this would be a loan payment. AKA arrearage.
Legal Definition
Debt that has accumulated and that has not yet been paid upon the due date. See arrearages.
Legal Definition
Moneys which are past due.
-- Ballentine's Law Dictionary