1. A subject, topic, or central idea that has not been identified, recognized, or is unfamiliar to the observer
The researcher struggled to identify the unknown theme in the ancient manuscript.
O pesquisador teve dificuldade em identificar o tema desconhecido no manuscrito antigo.
2. In literature or art, a recurring motif or concept whose origin or meaning is unclear
Critics debated the unknown theme that appeared throughout the artist's paintings.
Críticos debateram o tema desconhecido que aparecia em toda a série de pinturas do artista.