Synodic period is the period of time for a solar system body (e.g., a planet) to complete its orbit (orbital period), returning to the same position in relation to the Sun as seen from Earth. The relative position it returns to is not necessarily exact given the differences in orbital planes, orbital eccentricity, etc.. The Moon's synodic period is the period between times it is exactly full, or exactly new. For a planet beyond Earth's orbit (superior planet), it would be the period between occasions when the planet is opposite the Sun from Earth.
The alternative is an orbit's sidereal period, when the body returns to the same position in relation to the stars, which better matches the concept of "once around the orbit".