Definition of alter ego noun from the Oxford Advanced American Dictionary

     

    alter ego

     noun
    noun
    NAmE//ˌɔltər ˈiɡoʊ//
     
    (pl. alter egos)(from Latin)
     
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  1. 1 a person whose personality is different from your own but who shows or acts as another side of your personality Superman's alter ego was Clark Kent.
  2. 2a close friend who is very like yourself
See the Oxford Advanced Learner's Dictionary entry: alter ego