One who is a registered voter and politically informed, but is basically indifferent and non-committal toward the popular political parties (Democrat or Republican in the US). Probably believes that the political parties are so ideologically similar and so mired in their own bureaucracy that they have outlived their usefulness. Voting straight-ticket out of allegiance to a political party would be considered heresy.
"Are you a Democrat or a Republican?"
"Neither, I'm a political agnostic."