[Mr Skinner]

I don't quite agree, so I googled it:

ag·nosti·cal·ly adv.
Word History: An agnostic does not deny the existence of God and heaven but holds that one cannot know for certain whether or not they exist. The term agnostic was fittingly coined by the 19th-century British scientist Thomas H. Huxley

So by this definition which I share, it all has to do with belief:

You belive in god, you're a beliver (has nothing to do with proof!), you're not an agnostic.

You deny the possibility of there being a god (still no proof), you're an atheist.

You say there might be a god or a higher power we don't understand, but don't belive in a specific god, you're an agnostic.