meson

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. Any of a family of subatomic particles that participate in strong interactions, are composed of a quark and an antiquark, and have masses generally intermediate between leptons and baryons. See Table at subatomic particle.

Century Dictionary and Cyclopedia

1. n. The median plane which divides a body into two equal and symmetrical parts; the vertical longitudinal middle plane, dividing the body into right and left halves. Every median line lies in the meson. The dorsal border of the meson is called the dorsimeson; the ventral, ventrimeson. Also mesium. See median, adjective
2. n. See tetrachord.

Wiktionary

1. n. The mesial plane dividing the body into similar right and left halves.
2. n. obsolete A member of a group of subatomic particles having a mass intermediate between electrons and protons. (The most easily detected mesons fit this definition.)
3. n. now specifically, physics An elementary particle that is composed of a quark and an antiquark, such as a kaon or pion. (Meson composed of rarer quarks are much heavier.)

GNU Webster's 1913

1. n. (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.
2. n. (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 (π-meson), kaon (K-mesons), and η-mesons.

WordNet 3.0

1. n. an elementary particle responsible for the forces in the atomic nucleus; a hadron with a baryon number of 0