1An item accidentally left behind by its owner, often referring to personal belongings left on a train, in a shop, etc.
バスの中に傘の忘れ物があったと運転手さんが教えてくれた。
The bus driver told me there was an umbrella left behind on the bus.
学校の忘れ物は、職員室に届けることになっている。
Lost items at school are supposed to be turned in at the staff room.
ホテルのフロントで忘れ物を預かってもらった。
I had the hotel front desk hold on to a lost item for me.
映画館で忘れ物のスマホを届けてもらえて助かった。
I was relieved when someone turned in my phone that I'd left at the movie theater.
2The act (or resulting state) of forgetting or leaving something behind, used in expressions like 忘れ物をする ("to leave something behind").
出かける前に忘れ物がないか、もう一度確認した。
I checked once more before leaving to make sure I wasn't forgetting anything.
うっかり者の弟は、よく忘れ物をして先生に叱られる。
My absent-minded younger brother often leaves things behind and gets scolded by the teacher.
忘れ物をしないように、玄関にチェックリストを貼っている。
I've put up a checklist by the front door so I don't forget anything.