(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.)

plural of meson

Of or pertaining to mesons

(physics) The study of mesons.

(dated) A muon (kind of subatomic particle).

The upsilon particle

(physics) Either of two mesons made of a mixture of up, down and strange quarks and their antiquarks

(archaic, formal) A pion, a type of meson

plural of mu-meson