While I hesitate to get too pedantic about definitions, the way I tend to look at this is a little different. Theist/atheist refer to belief or lack of belief

While I hesitate to get too pedantic about definitions, the way I tend to look at this is a little different. Theist/atheist refer to belief or lack of belief in a deity or deities. Gnostic/agnostic refer to one's conviction of knowledge that there is or isn't a deity or deities. In my personal case, I don't claim to know for sure that there are no gods but I don't believe that there are so I'd call myself an agnostic atheist. Others I've met say "I believe that there is a god and I know this to be true" so I'd call them gnostic theists. Still others say with conviction that they know there just aren't any gods so they'd be gnostic atheists. Again, I don't claim that this is the only way these terms can be used or are used in practice but it works for me.