Prehensile tail
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A prehensile tail is the tail of an animal that has been adapted for use as a "fifth hand". Fully prehensile tails can be used to hold and manipulate objects, and in particular to aid arboreal creatures in finding and eating food in the trees. If the tail cannot be used for this it is considered only partially prehensile - such tails are often used to anchor an animal's body to or dangle from a branch, or as an aid to climbing.
Among most animals the prehensile tail is a New World adaptation, particularly in the fully developed form. Thus many monkeys such as howler monkeys and spider monkeys have grasping tails (unlike any of their distant cousins), and among the marsupials only the opossum has the (partial) ability. The only Old World animals with fully prehensile tails are the carnivorous binturongs of Borneo, though even they use only the tip of the tail. The kinkajou is often described as the only other carnivore to sport the adaptation, though its diet is actually omnivorous.
The capuchin monkey, South American porcupines, and several kinds of skink all have partially prehensile tails. It is interesting to note that the capuchin is more than intelligent enough to make use of a fully prehensile tail, but since the tail lacks an area of bare skin for a good grip it is only used in climbing and dangling. Other reasons for partial prehensility might include the lack of strength or flexibility in the tail, or simply having no need to manipulate objects.
The term prehensile is sometimes incorrectly used when "agile" is meant; but the term means "able to grasp" (from the Latin prehendere, the root of "comprehend" and "apprehend").