A monkey is a primate of the Haplorhini suborder and simian infraorder, either an Old World monkey or a New World monkey, but excluding apes and humans.[1] By this definition, the most common in biology, the monkeys are the group of all primates that are not tarsiers, lemurs, apes or humans and consist of about 260 known living species. Many species are arboreal, although there are species that live primarily on the ground, such as baboons. Monkeys are generally considered to be intelligent. Unlike apes, monkeys usually have tails. Tailless monkeys may be called "apes", incorrectly according to modern usage; thus the tailless Barbary macaque is called the "Barbary ape".

The New World monkeys (superfamily Ceboidea) are classified within the parvorder of Platyrrhini, whereas the Old World monkeys (superfamily Cercopithecoidea) form part of the parvorder Catarrhini, which also includes the hominoids (apes, including humans). Thus, as Old World monkeys are more closely related to hominoids than they are to New World monkeys, the monkeys are not a unitary (monophyletic) group.