In common parlance, it's a rebel or someone who goes their own way.
My pal Brenda told me about this NPR feature that gave the derivation of the term "maverick."
Seems there was a guy named "Maverick" (just like there was one named "Boycott" — didja know that?).
Samuel Maverick didn't want to brand his cattle (it was Texas in the 1800s and he had cattle, naturally) because he said it was cruel to the animals.
But other ranchers accused him of just wanting to claim all the unbranded cattle as his own.
Interesting, in light of the fact that one presidential candidate is trying to co-opt the other's theme, "Change."
So John McCain IS a maverick.