Unpaid invoices are considered outstanding.
Invoices are always issued with terms of payment. These terms vary widely from 'Cash terms', meaning that the invoice is due immediately, to many forms of 'Credit terms' (for example 30 days from date of invoice).
Invoices which remain unpaid for periods longer than their 'terms' indicate are considered overdue.
It is the aim of the Cash collection function of a business to collect Monies for all outstanding invoices before they become overdue and to mediate payment arrangements to ensure that invoiced debts do not become doubtful or bad.