The term 'highly unusual' can be described by four crossword solutions: ALIEN (5 letters), EXTRAORDINARY (13 letters), RARE (4 letters), and FREAK (5 letters). ALIEN refers to something that is not native to a particular place or environment, while EXTRAORDINARY implies something that is beyond what is usual or expected. RARE suggests something that is not commonly found or experienced, and FREAK implies something that is strange or bizarre. All of these words together describe something that is highly unusual.