What is the pouch of a marsupial called?

The pouch of a marsupial is called a marsupium.

The word marsupium comes from Latin for “pouch” and gives marsupials their name. In most marsupial species, a female’s marsupium is a fold of skin containing the teats, where a highly undeveloped newborn continues growing after birth.

Marsupial pregnancy is short compared with placental mammals. A newborn joey, often blind, hairless, and tiny, crawls from the birth canal to the pouch and attaches to a teat. It may remain attached or return to the pouch for protection while its organs, limbs, and senses develop. The exact shape and opening of the marsupium differ among species: some pouches open forward, while others open backward or are reduced.

“Pouch” is the everyday term, while marsupium is the anatomical term. Not every marsupial has a deep, obvious pouch, and the structure is not exclusively female in every case: males of a few species have a pouch-like fold. Monotremes also lack a true marsupium, even though the word “marsupial” is sometimes loosely associated with any mammal that carries young externally.