The refusal to choose between believing in a god and not believing.

Agnosticism attempts to find a middle ground between atheism and theism. It tries to claim that the existence or non-existence of god is unknowable, and therefore we can't make a choice.

If understood correctly, theism means a belief in god, where atheism means a non-belief. At any one time, you either believe or do not believe. There is no possible middle ground. A belief with some doubts is still a belief. A non-belief, while leaving open the possibility of future belief, is still non-belief. Agnosticism attempts to be on both camps, or neither camp. It is a contradictory idea, though. You must either believe or not believe. Any doubts in either direction don't change that requirement.

In practice, people who have stopped believing in god, but aren't confident enough to state that they don't believe, find comfort in the middle ground of agnosticism. Then they can claim that they're just not sure. It saves them from possibly offending or completely going against their theistic friends, while distancing themselves from their real position.