chariklo, i'm sorry, but this is not about a language barrier. i live in europe as well, and my first language isn't english (not even my second language, but wth), still, the common definition of atheism is simply not believing in any god. in the UK as well as in the US or anywhere else.
and opposed to what CA said, there are different flavors of atheism. there's "strong" atheism and there's "weak" atheism. a "strong" atheist would say "no god exists" while a "weak" atheist says "i don't believe that a god exists". this is getting mixed up, because some gods can be proven to not exist (such as the common god of the bible or quran), while other made up gods can't be proven to not exist. so most people (including christians and other believers) are strong atheists in some hindsight. but talking generally, most atheists are in the "weak" category. that is: if compelling evidence that (a) god exists is provided, they'd change their view, but as long as no evidence whatsoever exists, they won't believe in such a thing.
agnosticism is a whole different animal. theism/atheism is about believing or not believing. gnosticism/agnosticism is about being able to know or not. an agnostic says: i don't think one can know god. many atheists are agnostics as well. but you might be a believer and be an agnostic as well. this is a whole different story.