na-adjective, noun which may take the genitive case particle 'no', noun
unknown; unidentified; unspecified
Used in formal or written contexts to indicate that details such as a person's identity, origin, or date are not known or not given. Often appears in reports, news articles, and official documents.
犯人の身元は不詳のままだ。
The identity of the perpetrator remains unknown.
この書類には日付が不詳と記
The date is listed as unknown on this document.