any of the planets whose orbit lies inside the earth's orbit

a planet whose orbit is inside the Earth's orbit around the Sun: Mercury and Venus

A planet with an orbit lying closer to the Sun than the Earth's. The inferior planets are Mercury and Venus. Inferior planets are always close to the Sun and so are difficult to observe. They can be seen as evening and morning stars. Venus is the brighter of the two but Mercury is also a clearly visible unaided eye object. See also superior planets.