There's a couple definitions available.

Colloquial Agnostic (everyday usage)
This version basically means one who does not know if there's a god or not.

Agnostic (technically, what it is supposed to mean)
This would be one who believes that the god is unknowable, or univestigatable.

In reality, a colloquial agnostic is technically an atheist.

Differences exist between an agnostic and an atheist, although they aren't mutually exclusive.