The term 'EXTREMELY SUCCESSFUL' can be summarized by two crossword solutions: 'HUGE' (4 letters) and 'BOFFO' (5 letters). 'HUGE' is an adjective that means very large in size or amount. It is often used to describe something that is impressive or successful. 'BOFFO' is an informal adjective that means extremely successful or popular. It is often used to describe a movie, play, or other entertainment that has been very successful.