The term LEAST KNOWN can be defined as the adjective form of the word 'UNKNOWN', meaning something that is not widely known or recognized. It is often used to describe something that is not well-known or popular, such as a person, place, or thing. The crossword solution for this term is RAREST, which is another adjective form of the word 'RARE'. This word is used to describe something that is uncommon or scarce, and is often used to describe something that is not widely known or recognized.