A monkey is a primate of the Haplorrhini suborder and simian infraorder, either an Old World monkey or a New World monkey, but excluding apes and humans. There are about 260 known living species of monkey. Many 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.
Historical and modern terminology
According to the Oxford English Dictionary, the word «monkey» may originate in a German version of the Reynard the Fox fable, published circa 1580. In this version of the fable, a character named Moneke is the son of Martin the Ape. In English, no very clear distinction was originally made between «ape» and «monkey»; thus the 1910 Encyclopædia Britannica entry for «ape» notes that it is either a synonym for «monkey» or is used to mean a tailless humanlike primate. Such ambiguities persist. Colloquially, the terms «monkeys» and «apes» may still be used interchangeably. Due to its size (up to 1 m/3 ft) 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, such as the Barbary ape.
Later in the first half of the 20th century, the idea developed that there were trends in primate evolution and that the living members of the order could be arranged in a series, leading through «monkeys» and «apes» to humans. Monkeys thus constituted a «grade» on the path to humans and were distinguished from «apes».
Scientific classifications are now more often based on monophyletic groups, that is groups consisting of all the descendants of a common ancestor. The New World monkeys and the Old World monkeys are each monophyletic groups, but their combination is not, since it excludes hominoids (apes and humans). Thus the term «monkey» no longer refers to a recognized scientific taxon. The smallest accepted taxon which contains all the monkeys is the infraorder Simiiformes, or simians. However this also contains the hominoids (apes and humans), so that monkeys are, in terms of currently recognized taxa, non-hominoid simians.
A group of monkeys may be referred to as a mission or a tribe.