What is a 'buffalo soldier'?

A very quick Google would have scored the Wiki as the first hit:

Buffalo Soldiers originally were members of the 10th Cavalry Regiment of the United States Army, formed on September 21, 1866, at Fort Leavenworth, Kansas. This nickname was given to the Black Cavalry by Native American tribes who fought in the Indian Wars. The term eventually became synonymous with all of the African-American regiments formed in 1866.

Sources disagree on how the nickname "Buffalo Soldiers" began. According to the Buffalo Soldiers National Museum, the name originated with the Cheyenne warriors in the winter of 1877, the actual Cheyenne translation being "Wild Buffalo". However, writer Walter Hill documented the account of Colonel Benjamin Grierson, who founded the 10th Cavalry regiment, recalling an 1871 campaign against Comanches. Hill attributed the origin of the name to the Comanche, due to Grierson's assertions. The Apache used the same term ("We called them 'buffalo soldiers,' because they had curly, kinky hair ... like bisons") a claim supported by other sources.[3][4] [5] [6] Another possible source could be from the Plains Indians who gave them that name because of the bison coats they wore in winter.[7] The term Buffalo Soldiers became a generic term for all black soldiers. It is now used for U.S. Army units that trace their direct lineage back to any of the African-American regiments formed in 1866.