exercise ball

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A usually large, inflated plastic or rubber ball used especially for strengthening muscles and improving posture and balance, typically by leaning against or sitting on the ball and assuming different positions or by maneuvering the ball without using the hands.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A soft, elastic ball with a diameter of approximately 35 to 85 centimetres, used in physical therapy and exercise.