I'd have put it that agnostics believe that the existence of god cannot be (or has not been) either proved or disproved. This is probably the proper scientific position.
Don't forget the most popular religion by far: Apathyism. No box for that on the census form last time...
'Agnostic' comes from the Greek 'a', meaning 'an absence of', and 'gnostos' meaning 'known' or 'knowable'. Their viewpoint, in other words, is that any god is unknown/unknowable.