noun
outstanding; unsettled; unpaid
Used in accounting and finance for accounts, payments, or transactions that have not yet been settled or cleared.
未決済の請求書がまだ残っている。
There are still outstanding invoices.
未決済取引を確認してください。
Please check the unsettled transactions.
決済 means 'settlement' or 'payment completion'; 未決済 is its negative form indicating the settlement has not yet occurred.
Compound of 未 (mi, 'not yet') + 決済 (kessai, 'settlement, payment').