The orbital period is the time taken for a body to go once around a closed orbit. The orbital period of a planet is its "year".
For a body in an elliptical orbit of semimajor axis a about a much more massive body of mass M the orbital period P is given by
P = (2πa3/2) / √(gM).
This is the mathematical formulation of the third of Kepler's laws of planetary motion.