What is an atheist?

How do we define the terms theist, atheist and agnostic?
More importantly how do we use them.

The theist - believes that there is sufficient evidence that a deity exists and information about that God is knowable.

The atheist - believes that there is insufficient evidence for a diety and therefore non-exists.

The agnostic - believes that the evidence does not allow for a conclusion to be reached with certainty.

I use the word belief deliberately because each explores the same evidence and draws a different conclusion. This means to me that the choice is not made on rational grounds, in many ways the terms are not equivalent. There are many Christians for example that adhere to the faith, follow it's teaching but don't actually believe in a deity.

I would add that people move from one poition to another during their lives and so no person is locked into a position. I would also add that these views do not presuppose any particular view of any one religion.

What do people think?