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).  Also known as a christogram or chrismon.

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