Matt Taibbi provides as good a definition as I’ve read thus far.

“Vast forests have already been sacrificed to the public debate about the Tea Party: what it is, what it means, where it’s going. But after lengthy study of the phenomenon, I’ve concluded that the whole miserable narrative boils down to one stark fact: They’re full of shit. All of them. At the voter level, the Tea Party is a movement that purports to be furious about government spending — only the reality is that the vast majority of its members are former Bush supporters who yawned through two terms of record deficits and spent the past two electoral cycles frothing not about spending but about John Kerry’s medals and Barack Obama’s Sixties associations. The average Tea Partier is sincerely against government spending — with the exception of the money spent on them. In fact, their lack of embarrassment when it comes to collecting government largesse is key to understanding what this movement is all about — and nowhere do we see that dynamic as clearly as … in Kentucky, where Rand Paul is barreling toward the Senate ** with the aid of conservative icons like Palin.
“Early in his campaign, Dr. Paul, the son of the uncompromising libertarian hero Ron Paul, denounced Medicare as ‘socialized medicine.’ But this spring, when confronted with the idea of reducing Medicare payments to doctors like himself — half of his patients are on Medicare — he balked. This candidate, a man ostensibly so against government power in all its forms that he wants to gut the Americans With Disabilities Act and abolish the departments of Education and Energy, was unwilling to reduce his own government compensation, for a very logical reason. ‘Physicians,’ he said, ‘should be allowed to make a comfortable living.’ ”
— Matt Taibbi, “Tea and Crackers,” Rolling Stone, Oct. 15, 2010 issue.

** [Ed. Note: Rand Paul’s Democratic opponent, Jack Conway, has come up in the polls by 30 points in the past few months. He and Paul are now is a statistical dead heat.]