1. (astronomy) of, relating to, or determined by the joint action of two bodies, especially celestial bodies, moving in orbit
   "The synodic period of the Moon is the time between two consecutive new moons as seen from Earth"
    
2. Of, relating to, or produced by a synod (an ecclesiastical council)
   "The synodic decree was issued after lengthy deliberations by church leaders"

Encyclopedia: Synodic