The term 'REALLY POPULAR' can be represented by two crossword solutions: BIG (3 letters) and REDHOT (6 letters). BIG is an informal adjective used to describe something that is large in size, amount, or degree. It is often used to emphasize the magnitude of something. REDHOT is an informal adjective used to describe something that is very popular or fashionable. It is often used to emphasize the level of popularity or trendiness.