Heres a better definition: Any show made by Nick in the 90's that was cancelled due to the "Nicktoon Revolution" (the time when Nick started running "marathons for ratings") (90% of the classics were cancelled on May 23rd, 1999). So pretty much any show that aired on Nick before the year 2000 is a "Nick Classic"
Some of the most popular classics are: Salute Your Shorts, Pete & Pete, Clarissa, Hey Dude, Are You Afraid Of The Dark etc.