A person who either:
a. Does not have any theistic beliefs of any nature ("weak"
atheism)
b. Actively believes that there is no
God, or indeed any matter of supernatural entity ("strong"
atheism"
There are two noteworthy things about the concept of
atheism:
a. "Atheism", or a- in front of theism, simply means "not theistic". Therefore, the literal definition of the word atheism is what was referred to before as weak atheism. The differentiation between "weak" and strong atheism, however, is now necessary, due to the proliferation in use of labeling strong atheism as simply atheism.
b. "Belief" that there is no
God is still the important point here for strong atheism. No reasonable atheist would assert that they KNOW there is no
God, just as no reasonable theist would assert that they KNOW there IS a
God - there is no proof for either preposition, and thus certainty is impossible either way. However, strong atheism relies on the fact that logically proving something does NOT exist is far harder than proving something does exist (and in
fact is generally impossible), and thus tends not to perceive a lack of absolute proof that there is no
God as a weakness in their arguments.
I am an atheist. This tells you nothing about my personality, character, or
intelligence. All it tells you is that I do not have particular belief in
God. That is all that atheists have in
common.