noun

• (rare outside entomology) The mesial plane dividing the body into similar right and left halves.
• (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, 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.)