There are three categories of primates: prosimians, apes, and monkeys. Prosimians are a small group comprising lemurs, lorises, bushbabies, aye-ayes, tarsiers, and a few others. Apes are another small group containing larger species of primates such as chimpanzees, bonobos, gorillas, humans, orangutans, and gibbons. Monkeys include every species of primate that is not an ape or a prosimian.
Given these classifications, it’s incorrect to refer to chimpanzees or gorillas as monkeys. They’re apes.
Of course, the misuse monkey in place of ape is so common—and it has been for a long time—that we should probably just accept it. Still, the differences between monkeys and apes are pretty straightforward—and the small number of ape species makes them easy to remember—so we should be able to get this right.
