A possum (plural form: possums) is any of about 70 small- to medium-sized arboreal marsupial species native to Australia, New Guinea, and Sulawesi (and introduced to New Zealand and China). The name derives from their resemblance to the opossums of the Americas (the name is from Algonquian wapathemwa, not Greek or Latin, so the plural is possums, not possa or possi).
Possums are quadrupedal diprotodont marsupials with long tails. The smallest possum, indeed the smallest diprotodont marsupial, is the Tasmanian pygmy possum, with an adult head-body length of 70 mm (2 3⁄4 in) and a weight of 10 g (3⁄8 oz). The largest are the two species of bear cuscus which may exceed 7 kg (15 lb 7 oz). Possums are typically nocturnal and at least partially arboreal. The various species inhabit most vegetated habitats, and several species have adjusted well to urban settings. Diets range from generalist herbivores or omnivores (the common brushtail possum) to specialist browsers of eucalyptus (greater glider), insectivores (mountain pygmy possum) and nectar-feeders (honey possum). source