Since forever, people have subjugated each other to demand labor or resources. A monarchic or centralized power that subjugates several other peoples is an empire.
Half a dozen European empires expanded into the Americas during the Age of Exploration. With the help of infectious disease, these powers defeated lots of local tribes, and some larger groups. Best known are Spain's conquests of the Aztec empire in Tenochtitlán and the Incan empire in Cuzco.
Did other American empires also fall to the Europeans?
Some specific exclusions as I understand them: the Iroquois Confederacy was an important power but association in it was voluntary. The Taíno and Tupi cultures were widely distributed but had no central government to conquer. The Chimú empire was conquered by the Incas shortly before Europeans arrived.