bosu ball
n.— «One of my discoveries was something called a bosu ball, which looks like a stability ball chopped in half. Bosu is short for both sides up, and it gives old-fashioned exercises like push-ups a whole new level of difficulty, which again, is ultimately good for your core strength.» —“Have A Ball While Getting Fit For Ski Season” by Lachlan Mackintosh Vue Weekly (Edmonton, Alberta, Canada) Nov. 23, 2006. (source: Double-Tongued Dictionary)