Big ass is a colloquial expression for a large behind and it refers to a special preference of a bit more corpulent bottom. However, it has to be defined when a behind can be classified as a big ass. Preferences can range from a bit larger and well-proportioned behinds to rather enormous buttocks of primarily corpulent women. Usually women with behinds between these two extremes are preferred while good proportions are of importance.

Male fantasies can be obsessed by female asses. This preference has been realized by the erotic industry and has been marketed. Thus, various telephone sex providers, porn producers, and brothel owners have specialized on big asses. However, the different branches trigger different desires and fantasies.