When something, usually computer technology, is immensely outdated or lagging behind in graphics and/or other features so that it resembles the early versions of the America Online browser.
It can also be used when referencing old television shows, movies, car models, etc. and can be conveyed in either a positive or negative light depending on what is being discussed.
Steve: I just got finished playing Half Life!
Bill: Man, that game is sick; isn't Ravenholm creepy?
Steve: What? I was playing Half Life 1.
Bill: Are you serious? That's AOL4.0ldschool, man.