exercise ball
A very large, usually inflatable ball between 14 to 34 inches (35 to 85 cm) in diameter that is used in various exercises. These exercises are usually to strengthen the back, pelvis, and abdominal muscles.
Known by various names, such as: balance ball, birth ball, body ball, fitness ball, gym ball, gymnastic ball, physio ball, pilates ball, Pezzi ball, stability ball, Swedish ball, Swiss ball, therapy ball, Bosu ball, or yoga ball.
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This tag implicates ball (learn more).