What is the X in X-mas?
c.1600, "resembling a mitre," from French mitral, from Modern Latin mitralis, from Latin mitra (see mitre). Mitral valve is attested from 1705.
mitral mi·tral (mī'trəl)adj.
Relating to a mitral valve.
Shaped like a bishop's miter. Used of a structure resembling the shape of a headband or turban.