Diameter: Approximate distance across the equator, usually measured in miles or kilometers.
Eccentricity: In astronomy, a measure of how circular or elliptical an orbit is. A more elliptical orbit has a greater eccentricity.
Orbital Period: Average time required for the planet to go once around the Sun, or the moon to go once around its planet.
Retrograde: Most of the planets in our solar system, and the entire solar system itself, rotate counter-clockwise when viewed from the above the North Pole. Three planets are exceptions to this rule: Venus, Uranus, and Pluto. Scientists theorize that dramatic collisions may have caused these planets to rotate "backward" or have "retrograde rotation".
Period of Rotation: The time that it takes for the object to turn once around its axis. This is not exactly equal to the day length. For example, Earth turns around its axis once every 23 hours and 56 minutes. However, if we start measuring the rotation when our point on the Earth is exactly facing the Sun, it takes another 4 minutes (a full 24 hours) for us to exactly face the Sun again. This is because the Earth has moved along its orbit while it was turning -- it does not stay still in space and just turn around its axis.
Web page by Challe Hudson. Copyright 2001 Morehead Planetarium.