noun
(rare outside entomology) The mesial plane dividing the body into similar right and left halves.
noun
(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.)
(now specifically, particle physics) An elementary particle that is composed of a quark and an antiquark, such as a kaon or pion. (Mesons composed of rarer quarks are much heavier.)
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