/ ˌæltər ˈiːgəʊ; ˌɔːl-; NAmE ˈiːgoʊ/ noun ( pl. alter egos ) (from Latin )
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.
a close friend who is very like yourself
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WORD ORIGIN
mid 16th cent.: Latin , other self .