A possum is any of about 64 small to medium-sized arboreal marsupial species native to Australia, New Guinea, and Sulawesi (and introduced to New Zealand). The name derives from their resemblance to the opossums of the Americas and, unlike most names applied to Australian fauna in the early years of European colonisation, happens to be accurate: the opossums of America are distant relatives. (The name is from Algonquian wapathemwa, not Greek or Latin, so the plural is possums, not possa.) Possum is also used in North America as a diminutive for the Virginia Opossum. The possum's rank odour is due to its large musk glands located behind each ear.
Possums are small marsupials with brown or grey fur, ranging in size and weight from the length of a finger or 170 grams (6 ounces) (pygmy possums and wrist-winged gliders), to the length of 120 centimetres (four feet) or 14.5 kilograms (32 pounds) (brushtails and ringtails). All possums are nocturnal and omnivorous, hiding in a nest in a hollow tree during the day and coming out during the night to forage for food. They fill much the same role in the Australian ecosystem that squirrels fill in the northern hemisphere and are broadly similar in appearance.
The two most common species of possums, the Common Brushtail and Common Ringtail, are also among the largest.