A synodic day, also called a solar day, is the period it takes for a planet to rotate once on its axis, as viewed from the celestial body orbits (in the Solar System, this is the Sun). It is the basis of solar time.
The synodic day opposes the sidereale day, which is the period it takes for a planet ot complete rotation in relation to distant stars, which is the basis of sidereal time. This is different from the duration of a synodic day because the revolution of the body around its parent star would cause a single "day" to pass relative to a star, even if the body did not rotate itself.
Earth's synodic day[]
A synodic day on Earth is the time it takes for the Sun to pass over the same meridian (a line of longitude) on consecutive days, whereas a sidereal day is the time it takes for a given distant star to pass over a meridian on consecutive days. For example, in the Northern Hemisphere, a synodic day could be measured as the time taken for the Sun to move from exactly true south (i.e. its highest declination) on one day to exactly south again on the next day.
The synodic day on Earth is not constant, and changes over the course of the year due to the eccentricity of the Earth's orbit around the Sun, and the axial tilt of the Earth. The longest and shortest synodic days' durations differ by about 51 seconds. The mean length, however, is 24 hours (with fluctuations on the order of milliseconds), and is the basis of solar time. The difference between the mean and apparent solar time is the equation of time, which can also be seen in Earth's analemma.
As viewed from Earth during the year, the Sun appears to slowly drift along an imaginary path coplanar with Earth's orbit, known as the ecliptic, on a spherical background of seemingly fixed stars. Each synodic day, this gradual motion is a little less than 1° eastward (360° per 365.25 days), in a manner known as prograde motion.
Certain spacecraft orbits, Sun-synchronous orbits, have orbital periods that are a fraction of a synodic day. Combined with a nodal precession, this allows them to always pass over a location on Earth's surface at the same mean solar time.
Moon's synodic day[]
The Moon's synodic day is referred to as a Lunar day. Due to the Tidal locking\tidal locking with Earth, the Moon's synodic day (the lunar day or synodic rotation period) is the same as its synodic period with Earth and the Sun.
Venus's synodic day[]
Due to the slow rotational speed of Venus, its synodic rotation period of about 117 Earth days is about half the length of its sidereal rotational period (sidereal day), and even its orbital period.