What is a Christian?

It's simple, really.  Just as the word "Jungian" describes one who adheres to Psychologist Carl Jung's theories, and "Jacksonian" described followers of Andrew Jackson or his ideas, "Christian" describes a follower of Christ Jesus and his teachings.

The word "Christ" means the same as Messiah, which comes from the Greek word Christós, or "Anointed One."  We believe that the man Jesus is the Messiah foretold in the Old Testament. 

The word "Christian" was first used around A.D 46, in Antioch, a rather diverse and liberal-minded gentile city north of Israel.  It was applied toward those who followed Jesus, to differentiate them from all the other religions being practiced there. 

Nowadays, in common usage, "Christian" has a spectrum of meanings.

• At one extreme, a person might call themselves a Christian if, for example, they'd rather go to a church than any other house of worship, if they had to go at all, because that's what their parents or grandparents did.  They were born to Christian parents, grew up in a Christian home, so they think that makes them Christian.
• At the other extreme, others call themselves Christians because they have made an active, personal decision to wholly surrender their lives to following the teachings of Jesus.

A Christian is one who is on the way, though not necessarily very far along it, and who has at least some dim and half baked idea of whom to thank. — Frederick Buechner

Dim and half baked?  Well, when you consider how great God must be, to be the Creator of the Universe, I guess dim and half baked is about the best we can hope for, this side of Heaven.