What does the X in Xmas mean?

Originally the X was not an X as we use it in the English alphabet; it was the Greek letter chi which looks very similar to an X.

The Greek letter X (chi) is the first letter of the Greek Word for Christ, Xristos ( Χριστός) so Xmas therefore means "Christ's Mass" which is what the Word Christmas actually means.

For more information, please refer to the related question below.