Synodic day
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A synodic day is the period of time it takes for a planet to rotate once in relation to the body it is orbiting (as opposed to a sidereal day which is one complete rotation in relation to the stars).
Thus, a synodic day can be thought as 'sunrise to sunrise', whereas a sidereal day would be 'star-rise to star-rise'. These two quantities are not equal because the sun, as seen from earth, moves against the fixed sphere of stars along an imaginary line known as the ecliptic. The sun moves a little less than a degree each day -- (360 degrees/year)/(356.25 days/year) -- in East to West fashion known as prograde motion.
Although this thinking is correct, in a sense, neither term can be defined rising or setting of a celestial body because of variations due to the earth's tilt. See Equation of time.
For Earth, the Synodic day is known as a Solar day, and is 24hrs long.
