Definition of Pluto noun from the Oxford Advanced American Dictionary

 

Pluto

 noun
noun
NAmE//ˈplut̮oʊ//
 
 
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one of a number of round objects in space that are not as large as planets but that go around the sun. In August 2006, the International Astronomical Union declared that Pluto should be called a dwarf planet because it is smaller and has different characteristics from the other planets in our solar system; in 2008 it declared that dwarf planets farther from the sun than Neptune could also be called plutoids.
See the Oxford Advanced Learner's Dictionary entry: Pluto