Pronunciation enPR: ăs'tə-roid", IPA (key): /ˈæstəroɪd/ Audio (US)(file) Noun Singular asteroid Plural asteroids (countable) (astronomy) An asteroid is an object that is smaller than a planet that most often orbits a star, but it can sometimes orbit big planets. The asteroid orbited the star. The asteroid crossed over Jupiter's orbit. Wikiwand - on Seamless Wikipedia browsing. On steroids.Remove ads