The Collaborative International Dictionary of English v.0.48:

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]

WordNet (r) 3.0 (2006):

pion
    n 1: a meson involved in holding the nucleus together; produced
         as the result of high-energy particle collision [syn:
         pion, pi-meson]