The term 'prefix meaning 'EXTREME'' can be solved by two crossword solutions: ULTRA (5 letters) and ARCH (4 letters). ULTRA is derived from the Latin word 'ULTRA' which means 'BEYOND' and is used to describe something that is of the highest degree or quality. ARCH is derived from the Greek word 'arkhos' which means 'CHIEF' or 'RULER' and is used to describe something that is of the highest rank or authority. Both terms are used to describe something that is of the utmost degree or intensity.