The terms "inferior planet" and "superior planet" were coined by Copernicus to distinguish a planet's orbit's size; in relation to Earth.
"Inferior planet" is generally used in reference to Mercury and Venus; although it can be used to refer to any planet with a smaller orbit than some planet, X. Copernicus determined that these planets were, relatively, close to Sol, as they always appeared in its proximity. (See also: elongation)
"Superior planet" is generally used in reference to Mars, Jupiter, and Saturn, Uranus, Neptune, and Pluto; although, it can be used to refer to any planet with a larger orbit than some planet, X. Copernicus determined that Mars, Jupiter and Saturn were, relatively, far from Sol after noting that they were often on the opposite side of the celestial sphere (with repsect to Sol). Copernicus was not aware of the existence of Uranus, Neptune, or Pluto.