Someone who does not think there is enough evidence to prove or disprove God's existence. It's a valid viewpoint.
Some people say agnostics can't make up their mind or do their research on what to believe. I did my research on different religions and philosphical beliefs, and I made up my mind. And now I'm an agnostic. Deal with it.
Oh, and agnosticism isn't weak atheism. Atheism is believing that there is no God. You can be an agnostic athiest - one who doesn't believe in God. You can an agnostic deist - one who believes in a higher power. What you believe is different from what you know.
An agnostic is someone who does not think there is enough evidence to prove or disprove God's existence.