The term 'Five Nations Unit' refers to the Iroquois Confederacy, a powerful alliance of five Native American tribes located in the Northeastern United States. The five nations were the Mohawk, Oneida, Onondaga, Cayuga, and Seneca. The Iroquois Confederacy was formed in the 16th century and was a major political and military force in the region for centuries. The term 'TRIBE' is an appropriate crossword solution for this term as it accurately describes the members of the Five Nations Unit.