Under the Ritsuryo system (a system of centralized government based on the ritsuryo code), onshu was the registration of a person who came forward after having been missing from the family registers or the keicho (yearly tax registers), or after having been away from the place of registry.
陰首（おんしゅ）とは、律令制において戸籍や計帳から欠落している者及び浮浪の者が自主的に名乗り出て登録されること。