Semantics

buttocks
Мне не нравится её юбка — она обтягивает зад. I don't like her skirt: it strains across the buttocks.
rump
отвислый зад a droopy rump
butt
Мужик, а зад у него бабий. He's got a butt on him like a woman.
ass
Идёт и виляет задом, бесстыдница. She swings her ass when she walks, the shameless hussy.
This entry is concerned with the use of the word зад to refer to human buttocks and to the rear part of an animal. Common sense will dictate which of the expressions in this entry can or cannot be used to refer to animals or humans. The meaning "rear part of an object," e.g. зад повозки "tail of a wagon," is not described in this dictionary.