The amount outstanding on a debt.
For example, if you do not pay your rent, the debt still exists and is referred to as arrears.
Taxes not paid, due and payable and outstanding are tax arrears.
The same word is used to describe child support or spousal maintenance or support which is not paid by the due date.
In statutes, the term is used in the Ontario Family Responsibility and Support Arrears Enforcement Act (1996 SO Chapter 31 at canlii.org///on/laws/sta/1996c.31/index.html) not only in the title but also, for example, within:
"Any fees, costs, disbursements, surcharges or other charges charged by an assignee must be charged to the payor and may be added to the amount of arrears owing by the payor and may be collected in like manner as arrears."