Bubble butt is a slang term for a round, attractive backside. A bubble butt should have smooth, curved cheeks that protrude from the body slightly.
A bubble butt should not be confused with a large backside. While a bubble butt may be large, its roundness is its defining feature, not its size.
The term bubble butt is most commonly used in reference to a female’s backside, but it is occasionally used to describe a shapely male rear, especially one possessed by a homosexual male.