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
Emails with no subject are easily mistaken for spam.
無題 means 'untitled', the direct opposite of having a 題 (title) given to a work.
Compound of 無 (mu, 'without') and 題 (dai, 'title'). The reading is on'yomi.