noun
lost property report; reporting a lost item
Refers to the act of filing a report for a lost item, typically with the police or a station office.
財布を落としたので、交番に遺失届を出した。
I lost my wallet, so I filed a lost property report at the police box.
落とし物 is a more casual, everyday term for a lost item, whereas 遺失届 is a formal document or procedure.
Compound of 遺失 (loss, from 遺 'leave behind' and 失 'lose') and 届 (report, notification).