Whats a Christian?

I was talking to one of my friends, about christianity. The question of course came up, "Are you a Christian?" My friend replied, no i am not, but I am spiritual, and I do believe in God. For a second, i was caught off guard, and didn't know what to say. I tried not to be too closed minded and shut that entire theory/idea down. So I just didn't say anything.
My main question is, what is your definition of a Christian?
Also, can you be a Christian, without referring to yourself as a Christian?