Synodic day

A synodic day (or synodic rotation period or solar day) is the period for a celestial object to rotate once in relation to the star it is orbiting, and is the basis of solar time.

The synodic day is distinguished from the sidereal day, which is one complete rotation in relation to distant stars^[1] and is the basis of sidereal time.

In the case of a tidally locked planet, the same side always faces its parent star, and its synodic day is infinite. Its sidereal day, however, is equal to its orbital period.