The distinction between apes and monkeys is complicated by the traditional paraphyly of monkeys: Apes emerged as a sister group of Old World Monkeys in the catarrhines, which are a sister group of New World Monkeys. Therefore, cladistically, apes, catarrhines and related contemporary extinct groups such as Parapithecidae are monkeys as well, for any consistent definition of “monkey”. “Old World monkey” may also legitimately be taken to be meant to include all the catarrhines, including apes and extinct species such as Aegyptopithecus,[12][13][14][15] in which case the apes, Cercopithecoidea and Aegyptopithecus emerged within the Old World monkeys.

Source:en.wikipedia.org/wiki/Ape