A possum is a cat-sized nocturnal animal that's known for acting like it's dead when threatened. If you find a white animal with black eyes and ears in your garbage at night, it's probably a possum.
The word possum is an informal, regional variation on opossum that's only used in North America. It's especially common in the phrase "playing possum," which refers to the possum's involuntary reaction to fear or harm, falling into a death-like faint, and also to a person who pretends to be innocent. So you might accuse your younger brother, as he shrugs when you ask who ate the last brownie, of playing possum.