What exactly is a "Centrist"?

My friend Tully has a post up linking to another blog that's running a contest to come up with an alternate label for "centrists." Which is interesting in it's own right. But, it seems to me to beg the question of what exactly is a centrist in the first place.

Some would define it as an alternate name for a moderate who comes down on one definite side of the ideological center but who isn't as ideologically rigid as the more purist members of his/her party.

Others would define it as a description of an individual who holds views that are considered to come down on both sides and that when assigned a numerical value and added up yields a sum that is somewhere in the middle.

Still others would define it as an individual whose views are dominated by those which are inherently somewhere in the middle... with perhaps the odd non-centrist view still being held but not really changing the sum totality of political belief/opinion.

I personally am partial to that last definition while I largely reject the first definition because I don't see how not being an extremist necessarily makes one a centrist.

Post your definitions here or go over and join the thread at Tully's blog.