Definition of planet noun from the Oxford Advanced American Dictionary

      

    planet

     noun
    noun
    NAmE//ˈplænət//
     
     
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  1. 1[countable] a large round object in space that moves around a star (such as the sun) and receives light from it the planets of our solar system the planet Earth/Venus/Mars
  2. 2 the planet [singular] used to mean “the world,” especially when talking about the environment the battle to save the planet
  3. Idioms
    to be on another planet,what planet is somebody on?(informal)(humorous)
     
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    used to suggest that someone's ideas are not realistic or practical He thinks being a father is easy. What planet is he on?
See the Oxford Advanced Learner's Dictionary entry: planet