orbit:  The path an object in space travels as it revolves around another object in space is called an orbit.  One complete revolution around an object is also called an orbit.  The planets revolve around the Sun, and moons revolve around their planets, in elliptical (or oval-shaped) paths.  For some bodies, like Earth, the orbit is nearly circular.  For others, like Pluto, it's a more flattened ellipse.  The force of gravity is responsible for keeping planets, moons, and satellites in their orbits.  Johannes Kepler, in the late 1600s, was the first to describe in detail the orbits of the planets.