I was pondering this question because "success" often is better than "expected". You may expect 7-5 but consider 10-3 a success.
We look back on seasons, most of which have flaws, but often we see encouraging improvement in some aspect of the program.
One year my definition was getting lined up on defense ahead of the snap. It's tough to view a season with a conference championship and Rose Bowl win as anything but a success.