What exactly is a "fair share"?

With the looming fiscal bump, I've been waiting for this argument to reappear.  The most significant part of the fiscal cliff is the expiration of the Bush tax cuts.  It is clear to anyone with a pulse that some form of renewal of these cuts will have to be made--it is not likely to be bald renewal without significant change, but they will be renewed in one form or another.  Beohner and Obama will have to negotiate the details of which rates will remain the same and which rates will change, which loopholes will be closed and which left open.  Ultimately the argument will only concern the rich (in whatever way you'd like to define 'rich').  POTUS will ask that the rate for individuals making more than $250,000 be increased, and Boehner will have to determine how (actual rate increase, closing loopholes and deductions, raising rates on capital gains).  Even Bill Kristol, of the Weekly Standard, has argued that the GOP should give in here.  What drives me crazy is the vacuous use of the term (not to say concept--since I am claiming it is vacuous) "fair".  The rich must pay their "fair share" it will be argued by the left.  As it turns out, some on the right will make this claim as well.  And it is no doubt true.  The problem is that everyone assumes we all agree as to the meaning of fair, but no one actually defines it.  Some will claim that the rich should pay more because they have more.  Nevertheless, we'd never make the same claim about the cost of an iPad (I'm going to keep hammering away at Apple).  Apple cannot charge a different price depending on how much money you earn--that would be decidedly unfair.  Elizabeth Warren has argued that nobody has become rich on his or her own, that entrepreneurs are dependent on the state and federal highway systems for example.  This is no doubt true--but the rich are paying for these roads just as much as the poor (more even, one might argue).  And those who are paying more do not receive more for their money (the rich don't get to hog the road by virtue of having paid more in taxes).  In the article from which I have taken the paragraph at the beginning of this post, Wick Allison of the American Conservative can only find a meaning for the word fair in a Judeo-Christian morality which seems to be based largely on gut instinct.  But this has no more meaning than Warren's argument.  I am not suggesting that there is no way to make an argument based on fairness--but that argument has not been made yet.  I'd like to hear a solid argument of this sort, or I'd like a different argument altogether--perhaps one based not on fairness but on necessity.