Serifed version of Chi-Rho of Christian symbolism.
Originally uploaded by Chrismon in raster format, with the following summary:

The definitive example of a "chrismon" (christi monogramma). It is the combination of the Greek letters Chi and Rho, the first two Greek letters of "christos", referencing Jesus Christ. I created this image for the forthcoming Wikipedia entry about the Christian art known as "chrismons".
Originally from en.wikipedia; description page was here.
Original uploader was Jackaranga at en.wikipedia