A person who attends or graduated from Mount Holyoke College in South Hadley, MA. The college's main mascot is a "lyon," named after founder Mary Lyon, whose tomb can be found in the center of the campus.
(They are more commonly known as MoHos.)
Lyons are usually strong, liberal and intelligent women.
Person #1: Hey, our team lost to the Lyons last Saturday.
Person #2: Yeah, man, the Mount Holyoke team is pretty good.