The belief that it is not possible to know if there is or is not a God. (Compare with atheism that says there is no God, deism that says there is a God but he is not involved in the world, and theism which states that there is a God and he is involved in the world.) Agnosticism asserts the uncertainty of knowing if God exists.
The term was used by Thomas Huxley (1825-1895) to describe the inability to know many areas of knowledge, but has come to be associated most with the uncertainty of knowing God's existence.
Some consider agnosticism to be a category within atheism since agnostics, like atheists, do not affirm the existence of a god or gods while others consider it a middle ground between atheism (God does not exist) and theism (God exists as in active in the world).