a lost article; an item that has been lost or left behind — a formal term often used by police or public institutions.
駅の遺失物係に財布を届けた。
I handed a wallet in to the station's lost-and-found.
遺失物は一定期間保管されることが多い。
Lost items are often kept for a certain period.
交番で遺失物の届出をした。
I filed a report for a lost item at the police box.