They’re confused because too often we allow theists to define atheism according to their belief in absolutes. Since they absolutely believe their particular god exists, they assume that atheists must be just as absolute in their denial of the possibility of gods. They fail to appreciate that the only reason atheists don’t accept the arguments put forth by theists is because none of them are supported with any credible evidence. All they have to offer is faith, belief without substantive reason. Atheism doesn’t pretend to know that gods absolutely do not exist. We’re simply honest enough to admit the possibility of any god, let alone a specific god, existing as described by its followers is so improbable as to be statistically insignificant.

When it comes to definitions, I prefer to go with the reasoning put forth by the person who invented a word. ‘Agnostic’ was introduced by Thomas Henry Huxley in 1869.

When I reached intellectual maturity, and began to ask myself whether I was an atheist, a theist, or a pantheist; a materialist or an idealist; a Christian or a freethinker, I found that the more I learned and reflected, the less ready was the answer; until at last I came to the conclusion that I had neither art nor part with any of these denominations, except the last. The one thing in which most of these good people were agreed was the one thing in which I differed from them. They were quite sure that they had attained a certain “gnosis”–had more or less successfully solved the problem of existence; while I was quite sure I had not, and had a pretty strong conviction that the problem was insoluble. And, with Hume and Kant on my side, I could not think myself presumptuous in holding fast by that opinion …

[Quoted in Encylopaedia of Religion and Ethics, 1908, edited by James Hastings MA DD]

He also wrote,

Agnosticism, in fact, is not a creed, but a method, the essence of which lies in the rigorous application of a single principle. That principle is of great antiquity; it is as old as Socrates; as old as the writer who said, ‘Try all things, hold fast by that which is good’; it is the foundation of the Reformation, which simply illustrated the axiom that every man should be able to give a reason for the faith that is in him, it is the great principle of Descartes; it is the fundamental axiom of modern science. Positively the principle may be expressed: In matters of the intellect, follow your reason as far as it will take you, without regard to any other consideration. And negatively: In matters of the intellect, do not pretend that conclusions are certain which are not demonstrated or demonstrable. That I take to be the agnostic faith, which if a man keep whole and undefiled, he shall not be ashamed to look the universe in the face, whatever the future may have in store for him.

[" Agnosticism," 1889]

Atheism is, I think, rather clearly the lack of belief in gods, period. Since I lack a belief in gods, I can reasonably describe myself as an atheist. Yet in the pursuit of knowledge I employ and endorse agnosticism and skepticism. So I’m an agnostic in the quest for knowledge, but an atheist when it comes to belief in gods. I’m also an aNellyist when it comes to belief in the Loch Ness Monster and an aYetiist when it comes to belief in Big Foot.