It is the active belief that there is no God, based either on faith or on a lack of empirical evidence. But whether God(s) exist(s) or not is currently unprovable... so you're participating in an active belief.
You can be agnostic and accept that you don't know either way and not be particularly concerned about it... or you can identify as an atheist and be "certain".
I'm agnostic towards ghosts.
