Webster's English Dictionary

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meson

me.son \'mez-.a:n, 'me-z-, 'mes-, 'me-s-\ \me-'za:n-ik, me--, -'sa:n-\ n 
   [ISV mes- + -on] : an unstable nuclear particle first observed in cosmic 
   rays that has a ma ss typically between that of the electron and the 
   proton, that is either positively or negatively charged or neutral, and 
   that occurs in more than one variety - me.son.ic aj