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The Collaborative International Dictionary of English v.0.48 (gcide)

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]