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.
This piece was released untitled.
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.