Ivy League - Group of eight old, distinguished universities in the northeastern U.S., high in academic and social prestige (Harvard, Yale, Princeton, Columbia, Brown, and Cornell Universities, the University of Pennsylvania, and Dartmouth College). Also called the "Ancient Eight" or simply the "Ivies"
There are a myriad theories that revolve around this one. To wit:
The ivy in the term is referring to the ivy-covered walls of the historic buildings of these institutions (See image)
One theory leads us to believe that over a century ago, an interscholastic athletic league was formed that comprised of Harvard, Yale, Princeton, and a fourth school that keeps changing with the stories . It was officially called "IV League"; IV standing for Roman numeral four.
Most agree though that the term came from the sports world when New York Tribune sportswriter in 1933 who while describing the inter-university football season wrote, "A proportion of our eastern ivy colleges are meeting little fellows another Saturday before plunging into the strife and the turmoil".
Source: Wikipedia, Google Images