noun
owner of a lost article
Formal or legal term used in contexts such as lost-and-found procedures or property law.
遺失者が現れない場合、拾得者が所有権を取得することがある。
If the owner of the lost article does not appear, the finder may acquire ownership.
落とし主 is a more everyday term for the person who dropped or lost something, whereas 遺失者 is formal/legal.
Compound of 遺失 (loss, from 遺 'leave behind' and 失 'lose') and 者 (person).