Meson \Mes"on\, n. [NL., fr. Gr. me`son middle, neut. of
         me`sos,
   a., middle.]
   1. (Anat.) The mesial plane
         dividing the body of an animal
      into similar right and
         left halves. The line in which it
      meets the dorsal
         surface has been called the dorsimeson,
      and the
         corresponding ventral edge the ventrimeson. --B.
      G.
         Wilder.
      [1913 Webster]

   2. (Physics) An
         elementary particle made up of two quarks; a
      hadron
         having a baryon number of zero; any hadron other
      than
         a baryon. Mesons are bosons with integral values of
     
         spin, having a mass intermediate between those of the
     
         electron and a nucleon; they may have positive or negative

              charges, or may be neutral. Mesons are of three types:
         the
      pion
         ([pi]-meson), kaon
         (K-mesons), and
     
         [eta]-mesons.
    
          [PJC]

   mu meson
         (Physics) the
         former name for the muon,
         a
      particle which is not a
         true meson[2]. The term is no
      longer used in
         technical literature, except historically.
      [PJC]