A monkey's any cercopithecoid (Old World monkey) or platyrrhine (New World monkey) primate. All primates that aren't prosimians (lemurs and tarsiers) or apes are monkeys. The 264 known extant monkey species represent two of the three groupings of simian primates (the third group being the 21 species of apes). Monkeys are usually smaller and/or longer-tailed than apes. The New World monkeys are classified within the parvorder Platyrrhini, whereas the Old World monkeys (superfamily Cercopithecoi… (More on Monkey)