If I had to define it as narrowly as possible, I'd say it is someone who accepts, believes or knows that Jesus is divine.
From here you would have to further define orthodox, protestant, literal fundamentalist, etc. just as you would the differences in reform/orthodox judaism, theraveda/mahayana buddhism, Sunni/Shia muslim, etc.
It's more than knowing that Christ is divine. Satan also knows that Christ is divine, but Satan is not a Christian. It's choosing Christ as your savior.