noun which may take the genitive case particle 'no'
untitled; without a title; no subject (email)
Describes a work, message, or item that has no title. Common in art, music, and email contexts.
この曲は無題のまま発表された。
This piece was released untitled.
無題のメールは迷惑メールと間違えられやすい。
Emails with no subject are easily mistaken for spam.