NB The word задница is more vulgar than зад. It may be used as an epithet, e.g. Ах ты, задница! "You asshole!" The word зад is not used in such expressions. Thus there are also no neutral adjectives used with задница, while there are such used with зад, e.g. круглый зад "a rounded butt," худой зад "a skinny butt."

NB The words попа and попка are usually used to refer to that part of a child's anatomy: