Any South American monkey (platyrrhine) species in either of two families: marmosets (Callitrichidae) or South American monkeys other than marmosets (Cebidae), including capuchins and spider monkeys.
One of the hallmarks of being a hominid is having opposable thumbs: stronger, longer and more flexible than the thumbless hands of a spider monkey or the non-opposable thumbs of a marmoset.