planet
n. countablen. a very large, round object in space that moves around a star like the Sun. Earth is the planet we live on.
n. a celestial body that orbits a star, has sufficient mass for its own gravity to make it nearly round, and has cleared the neighborhood around its orbit.
Mars is known as the red planet.
Scientists are searching for another planet in our galaxy that might support human life.
The discovery of several rocky planets orbiting the nearby star has reignited public interest in the possibility of extraterrestrial biological signatures.
From Middle English planete, from Old French planete, from Latin planeta, planetes, from Ancient Greek πλανήτης (planḗtēs, “wanderer”) (itself an ellipsis of ἀστέρες πλανῆται (astéres planêtai, “wandering stars”)), from Ancient Greek πλανάω (planáō, “wander about, stray”), of unknown origin. Cognate with Latin pālor (“wander about, stray”), Old Norse flana (“to rush about”), and Norwegian flanta (“to wander about”). More at flaunt. So called because they have apparent motion, unlike the "fixed" stars. Originally including also the moon and sun but not the Earth; modern scientific sense of "world that orbits a star" is from 1630s in English. The Greek word is an enlarged form of πλάνης (plánēs, “who wanders around, wanderer”), also "wandering star, planet", in medicine "unstable temperature."
Often used with the definite article 'the' to refer specifically to Earth.