| The 六大 rokudai, or "Big 6," is a baseball league comprised of the top six universities in Japan. In a nutshell, it's the Japanese copy of the American Ivy League. Member schools are:
Hosei University
Keio University
Meiji University
Rikkyo University
University of Tokyo
Waseda University
Virtually everyone who's anyone in Japan goes to one of these colleges at some point in their lives: they are where the country's elite are bred.
The term "Rokudai" is usually only used when talking about college baseball itself: one difference between the rokudai and the Ivy League is that its teams are actually good. The home base for the rokudai is on the satellite grounds of the Meiji Shrine in Aoyama (Jingu Kyujo or "Shrine Field"), possibly the most famous stadium in Japan. Tickets are cheap, too: you can get box seats for 1,300 yen a pop.
Rokudai league website: http://www.big6.gr.jp/ | 
