A monkey is any member of either the New World monkeys or Old World monkeys, two of the three groupings of simian primates, the third group being the apes.
The New World monkeys are classified within the parvorder Platyrrhini, whereas the Old World monkeys form part of the parvorder Catarrhini, which also includes the apes. Thus, scientifically speaking, monkeys do not form a "natural group", in that the Old World monkeys are actually more closely related to the apes than they are to the New World species. There are 264 known extant species of monkey. Because of their similarity to monkeys, apes such as chimpanzees and gibbons are often called monkeys in informal usage, though biologists don't consider them to be monkeys. Conversely, due to its size (up to 1 metre); the Mandrill is often thought to be an ape, but it is actually an Old World monkey. Also, a few monkey species have the word "ape" in their common name. Because they are not a single coherent group, monkeys do not have any particular traits that they all share and are not shared with the remaining group of simians, the apes.