(Also known as a christogram or chrismon). A symbol for Jesus Christ used in Christian art. It is formed by superimposing the letters chi (X) and rho (P), the first two letters of ΧΡΙΣΤΟΣ (“Christ” in Greek).  

Chi-rho on an altar table (det.), from Khirbet Um El ’Ahdam, Algeria. Limestone. Third quarter of the 4th c. Musée du Louvre, Paris.