positronium
noun
pos·i·tro·ni·um ˌpä-zə-ˈtrō-nē-əm 
: a short-lived system that consists of a positron and an electron bound together and is suggestive of a hydrogen atom
Example Sentences
Recent Examples on the Web For example, in the 1990s physicists thought, essentially, that positronium didn’t live as long as predicted. 
Adrian Cho, Science | AAAS, 25 Aug. 2020 That makes positronium an ideal tool with which to look for new physics, Cassidy says. Adrian Cho, Science | AAAS, 25 Aug. 2020
Adrian Cho, Science | AAAS, 25 Aug. 2020 That makes positronium an ideal tool with which to look for new physics, Cassidy says. Adrian Cho, Science | AAAS, 25 Aug. 2020 Word History
First Known Use
1945, in the meaning defined above