The word opossum (usually pronounced without the leading 'O', or with only a very slight schwa) refers either to the Virginia Opossum in particular, or more generally to any of the other marsupials of magnorder Ameridelphia.
The Virginia Opossum is noted for its reaction to threat, which is to feign death. This is called "playing (o)possum".
Though opossum is often pronounced, and even spelled possum, the latter word actually refers to various distantly related but broadly similar animals of magnorder Australidelphia, that are native to Australasia and are an introduced pest in New Zealand. See possum.
See also: Pogo