I don't mind the definition of classic by SoBerious, but I'm a little more loose with it - for me, what I consider a classic album is a personal decision. I definitely agree with (1) - has to have withstood the test of time. For example:
A lot of people on this post consider Bone Thugs to have produced 'classic' albums, so for them they are classics. However, I've never really felt Bone Thugs music, so for me they're not classics.
I agree with several people when they say that Tupac - Strictly for my N.I.G.G.A.Z and Spice 1 - AmeriKKKs nightmare are classic albums, so these would be considered universally classic (by most of you anyway).
Several of the albums I consider classic (like M.O.P - Firing Squad and Paris - Sleeping with the Enemy) most people wouldn't consider classic. However, I grew up with them, and whenever I hear these albums my mind shoots back to those old days and the memories come rushing back. For me, that is what it means to be classic, whether anyone else agrees with me or not.
They dont' have to be the best of that time, or change the game in any way whatsoever, but as long as you loved the album back then and still love it now, then that by my definition is a classic.