meson

Short definition from Wiktionary

Noun

1. 1
   The mesial plane dividing the body into similar right and left halves.
2. 2
   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. 3
   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.)