B.O.U.O. (ball of unknown origin) is the red and grey ball that House owns. It is speculated that it is a tennis ball, or a dog toy. The ball has a needle hole for inflation or deflation.
House will entertain himself with the ball by throwing it to himself, bouncing it of a wall, flipping it around with his cane, etc. House uses it most when trying to think about his case, and in tough situations.
House has affectionately referred to it as "Ba Ba" and "Bally."