[myoo-oh-nee-uh m]
noun Physics.
  1. an electron and a positively charged muon bound together by electrical attraction in the same manner as the electron and proton in a hydrogen atom.

Origin of muonium

1955–60; mu(on) + -onium, probably extracted from positronium Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018