Dictionaries contain definitions of the common (status quo) usage of words, usually this is a good thing.
For instance, if you look up
inflammable you'll see that it's meaning is pretty much the same as flammable. Logically speaking, they should be opposites, but if keeping the definition of "inflammable" as flammable prevents people who don't know better from immolating themselves, that's a good thing.
The definition of atheism is like the definition of inflammable. Through some goofy linguistic inbreeding, these words came to mean something completely different from what they logically should mean.
In atheism's case, people need to knock some sense in to the status quo language. Using a meaningless definition of atheism handicaps people when it comes to accurately and clearly describing their, or other's, views about the existence of God.
Logically, the status quo definitions of atheism and agnosticism (where atheism is "God doesn't exist" and agnosticism is "I'm waiting for evidence") don't make sense. In fact, both of of those definitions represent completely untenable positions.
"Status quo atheism" e.g. believing that "God doesn't exist" is pointless, because it's (so far) impossible to prove.
Richard Dawkins happens to agree with me.
"Status quo agnosticism" e.g. believing that God could or could not exist, is meaningless. It's like making a bet that the person walking by either will or won't be wearing clothes. If that's all you believe, it's really not even worth saying. You have three options, a clothed person, a naked person, or you wait indefinitely for someone to walk by.
Agnosticism is really more of an epistemological question: If there is a God, can you know that it exists? Agnosticism says, so far, no.
So, all agnostics are atheists, but not all atheists are agnostics.
If the word atheism was used and understood correctly, more people would just say "Currently, I'm not a believer, and by the logical definition of the word, that means I'm not a theist; therefore I am an atheist. Of course, like everything else in the world, this is subject to change." That's really all there is to it. Atheism is about taking a position of
maximum parsimony, nothing else.