Which primate group is commonly called the lesser apes?

The primate group commonly called the lesser apes is gibbons.

Gibbons are the living apes of the family Hylobatidae, traditionally contrasted with the great apes: humans, chimpanzees, bonobos, gorillas, and orangutans. The term “lesser” refers mainly to their generally smaller bodies, not to inferior intelligence or evolutionary status. Gibbons are highly adapted to life in the trees and are famous for moving through forest canopies by brachiation, swinging beneath branches with their long arms.

More than a dozen living gibbon species are recognized, depending on the taxonomic treatment. They inhabit forests across South and Southeast Asia, including parts of India, Bangladesh, Myanmar, Thailand, Malaysia, Indonesia, and southern China. Their loud, far-carrying songs help advertise territories and maintain pair or family bonds.

Gibbons are still apes, so they do not have tails—an easy distinction from many monkeys. They are also not members of Hominidae, the great-ape family. The two ape lineages diverged millions of years ago, with gibbons representing the earliest living branch of the hominoid group.