The Gros Ventre (from French: big belly), also known as the A'ani or A'aninin (their autonym), and Atsina, are a historically Algonquian-speaking Native American tribe located in north central Montana.

There are currently 3,682 enrolled members.

The federally recognized tribe shares Fort Belknap Indian Reservation with the Assiniboine, their historical enemies.The French used the term Gros Ventre, which was mistakenly interpreted from their sign language.

Instead, the Gros Ventre people call themselves by the autonym A'ani or A'aninin, which means "white clay people," or more loosely "ourselves, our people, us, we," etc.The Arapaho and the A'aninin were a single, large Algonquian-speaking people who lived along the Red River valley in northern present-day Minnesota and Canada.

In the early 18th century, the large tribe split into two; forming the A'aninin and the Arapaho.

The A'aninin stayed in the Saskatchewan region, and the Arapaho went south.

Their languages became differentiated after that time.The Piegan Blackfoot, enemies of the Gros Ventre throughout most of history, called the A'aninin, "Piik-siik-sii-naa," which translates to "snakes."

According to the Piegan Institute, the contemporary Piegan name for the Gros Ventre is "Assinee," meaning "big bellies," which is similar to the falsely translated label applied by the French.

Atsina, a Pieagan word, translates to either "gut people" or "like a Cree."

Further clarification of the name is required.

After the division of peoples, the Arapaho, who considered them inferior, called them Hitúnĕna, meaning "beggars".

Other interpretations of the term have been "hunger", "waterfall", and "big bellies".