Which term best describes an ethernet system operating at 100mbps?

The term that best describes an Ethernet system operating at 100 Mbps is "Fast Ethernet." Fast Ethernet is a standard that allows for data transmission at 100 megabits per second, typically using twisted pair cabling (like Category 5) or fiber optics. It was widely adopted in the late 1990s and serves as an upgrade from the original 10 Mbps Ethernet.