Noun, Noun which may take the genitive case particle 'no'
registered mail; registered post; registration (of mail)
Abbreviationsee 書留郵便
この手紙は紛失するといけないから、書留にするがよい。You had better get this letter registered for fear it should be lost. Noun
(putting on) record; writing down; noting down; registry
Also written 書き留め【かきとめ】、書留め【かきとめ】