positronium

noun
pos·​i·​tro·​ni·​um | \ˌpä-zə-ˈtrō-nē-əm \

Definition of positronium 

: a short-lived system that consists of a positron and an electron bound together and is suggestive of a hydrogen atom

First Known Use of positronium

1945, in the meaning defined above

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The first known use of positronium was in 1945

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