orbital plane: All the points along a planet's orbit form a plane. This plane is called the planet's orbital plane. If you think of the planet's orbit as a disk instead of an ellipse, and then you imagine the disk extending infinitely through space, you have pictured the planet's orbital plane. The orbital planes of planets are not all exactly the same. For example, Mars's orbital plane is slightly tilted compared to Earth's orbital plane (which is called the ecliptic).
