The infraorder Simiiformes was named in...
Dichromatic (two-color) vision, inherited from basal primata, preserved in platyrhini. Modified to trichromatic (three-color) vision in catarhini.
The diagnostic features used to distinguish monkeys (simiiformes) from other simians (haplorhines), include:
Absent sensory whiskers
Single pectoral pair of mammae
Unsheathed, pendulous penis
Bony plate in back of orbit
Fully binocular vision
Increased encephalization (see neoteny, below)
More rounded cranium (see neoteny, below)
In addition, simiiformes are neotenous compared to other primates. This means that we retain a more juvenile appearance.
Williams, B. Kay, R. Kirk, C. New perspectives on anthrpoid origins. PNAS March 16, 2010 107 (11) 4797-4804; https://doi.org/10.1073/pnas.0908320107