is an archaic term that is sometimes used to refer to the planets Mercury and Venus. The name originated from the fact that they orbit closer to the Sun than the Earth and hence, in the geocentric cosmology of Ptolemy, both appeared to travel with the Sun across the sky. This is in contrast to so-called superior planets, such as Mars, that appeare...
A planet that orbits the sun inside earths orbit, these would be Mercury and Venus.
Either of the planets Mercury or Venus, whose orbits are closer to the Sun than is Earth`s orbit. Other planets are called superior planets.
Planet which orbits closer to the Sun than the Earth's orbit. 
A planet that orbits between the Earth and the Sun. Mercury and Venus are the only two inferior planets in our solar system.
[n] - any of the planets whose orbit lies inside the earth`s orbit
Planet (Mercury or Venus) whose orbit lies within that of the Earth, best observed when at its greatest elongation from the Sun, either at eastern elongation in the evening (setting after the Sun) or at western elongation in the morning (rising before the Sun)
either of the two planets whose orbits are inside the orbit of the earth: Venus and Mercury. Cf. superior planet.