I personally believe that whoever snuck in a definition for Atheism as "lack of belief in a God" is the biggest troll of all. If that line wasn't used as the secondary definition of Atheism in every readily available dictionary, half these semantics arguments could be avoided, and people who are unsure of the existence of a deity could go back to calling themselves Agnostics.
The semantic arguments are completely unnecessary, if people knew what they're talking about.
"A" means without. Theism means belief in a deity. A-Theism = without a belief in a deity. Why should have we made some exclusion to the rule for this specific word? And that is easier to define, than to try and define the god a specific person believes in; after all there are nearly 7 billion different ideas of God on Earth.
Agnosticism and atheism aren't mutually exclusive.
*Agnostic atheist (Weak atheist) = "I don't believe in a god, but I cannot disprove it either."
**Gnostic atheist (Strong atheist) = "I know there are no gods."
Undefined atheist (Atheist) = "I don't believe in a god"
Agnostic theist = "I believe in a god, but I cannot prove it exists."
Gnostic theist = "I know God exists."
Undefined theist = "I believe in a god"
Edit:* Disproving something that we cannot define is impossible.
** This is a logical stance, only if we're talking about a specific god, that can be disproven.