Inspired by @Conscious thoughts 's thread. Mine? I'm not even gonna touch a definition. Too hard to come up with one that encompasses all religions. If you say your belief system is a religion then it is one. I'm not gonna argue. Most atheists don't say they are a religion. Thus I don't believe they are a religion. Some Satanists say they are others say they are just a philosophy rather then a religion. I consider my beliefs religious tho some dont. I might have a definition but I find it hard to word it.
I tend to think of religion in terms of practice rather than belief. An atheist can be religious, and a theist can be non-religious.
I am both an atheist, and non-religious. I'd suggest that's the most common position amongst atheists.