unpaid taxes; tax arrears
Refers to taxes that have not been paid by the due date; used in administrative, legal, and financial contexts.
If you have unpaid taxes, late payment charges will accrue.
The city is focusing on collecting tax arrears.
延滞金 is the late payment penalty itself, whereas 未納税 is the unpaid tax amount.
Compound of 未 (mi, 'not yet') + 納 (nō, 'pay') + 税 (zei, 'tax'). The reading みのうぜい follows standard on'yomi.