The term GYM BALL can be solved by the four letter word PROM. PROM is an abbreviation for promenade, which is a FORMAL dance or gathering usually held at the end of a school year. It is also known as a 'FORMAL' or 'junior/senior PROM'. A gym ball is an exercise ball used in physical therapy and fitness training. It is typically made of rubber and filled with air, and is used to improve balance, coordination, and strength.