The Arapaho (; in French: Arapahos, Gens de Vache) are a tribe of Native Americans historically living on the plains of Colorado and Wyoming. They were close allies of the Cheyenne tribe and loosely aligned with the Lakota and Dakota. The Arapaho language, Heenetiit, is an Algonquian language closely related to Gros Ventre (Ahe/A'ananin), whose people are considered to have separated from the Arapaho at an early time. The Blackfeet and Cheyenne also speak Algonquian languages, but theirs are quite different from Arapaho.
Once established, the Arapaho began to expand on the plains through trade, warfare, and alliances with other plains tribes. Around 1811, the Arapaho made an alliance with the Cheyenne (Hitesiino' - ''scarred one''). Their strong alliance with the Cheyenne allowed the Arapaho to greatly expand their hunting territory. By 1826, the Lakota, Dakota, Cheyenne, and Arapaho pushed the Kiowa (Niiciiheihiinenno'; Kiowa tribe: Niiciiheihiiteen) and invading Comanche to the south. Conflict with the allied Comanche and Kiowa ended in 1840 when the two large tribes made peace with the Arapaho and Southern Cheyenne and became their allies. Chief Little Raven was the most notable Arapaho chief; he helped mediate peace among the nomadic southern plains tribes and would retain his reputation as a peace chief throughout the Indian Wars and reservation period. The alliance with the Comanche and Kiowa made the most southern Arapaho bands powerful enough to enter the Llano Estacado in the Texas Panhandle. One band of Southern Arapaho became so closely allied with the Comanche that they were absorbed into the tribe, adopted the Comanche language, and became a band of Comanche known as the Saria Tuhka (Sata Teichas) or Dog-Eaters band. Along the upper Missouri River, the Arapaho actively traded with the farming villages of the Arikara, Mandan, and Hidatsa, trading meat and hides for corn, squash, and beans. The Arikara referred to the Arapaho as the "Colored Stone Village (People)", possibly because gemstones from the Southwest were among the trade items. The Hidatsa called them E-tah-leh or Ita-Iddi (''Bison-Path People''), referring to their hunting of bison. Conflict with Euro-American traders and explorers was limited at the time. The Arapaho freely entered various trading posts and trade fairs to exchange mostly bison hides and beaver furs for European goods such as firearms. The Arapaho frequently encountered fur traders in the foothills of the Rocky Mountains, and the headwaters of the Platte and Arkansas. They became well-known traders on the plains and bordering Rocky Mountains. The name ''Arapaho'' may have been derived from the Pawnee word Tirapihu (or Larapihu), meaning "he buys or trades" or "traders". The Arapaho were a prominent trading group in the Great Plains region. The term may also have come from European-American traders referring to them by their Crow (Apsaalooke alilaau) name of Alappaho', which meant "People with many tattoos". By custom the Arapaho tattooed small circles on their bodies. The name Arapaho became widespread among the white traders.

Q: when was the expansion?
A: 1826,

Q: what expanded?
A: the plains
what was special about the expansion?
allowed the Arapaho to greatly expand their hunting territory. Around 1811, the Arapaho made an alliance with the Cheyenne (Hitesiino' - ''scarred one''). Their strong alliance with the Cheyenne allowed the Arapaho to greatly expand their hunting territory. Their strong alliance with the Cheyenne allowed the Arapaho to greatly expand their hunting territory. through trade, warfare, and alliances with other plains tribes.
through trade, warfare, and alliances with other plains tribes.