Dunno about definition but here is the Nicene Creed of retrobiking.
(apologies to the religious among you, but I came to the conclusion that the tenets of the Christian faith are more reasonable statements than most of these).
1) I believe I have the ability to fettle parts that weren't made to work together.
2) I believe can make worn out stuff work better than it used to.
3) I believe most doggedly that some things don't need to be improved upon, even the ones that do.
4) I believe that it is right to be proud of being an old fart.
5) I believe that I have too much time/space/money on my hands.
6) I believe in following the fashion that was in fashion when I was young enough to be interested in fashion. At least, in fashion.
7) I believe in the approval of my peers and have scant regard for that of the uninitiated.

I believe that more time should be spent fettling a bike than actually riding it.
9) I believe that it is right to do things the hard way, if possible.
10) I believe that the boundaries of good taste do not apply if they get in the way of a pipe dream.