(mes′on, mē′son) noun
1. The plane that divides the body longitudinally into two halves; the median or mesial plane.
2. Music Loosely, a tetrachord. [<NL <Gk. mesos middle]

(mes′on, mē′son) noun Physics
Any of a group of short–lived, unstable atomic particles having a mass intermediate between that of the electron and the proton. They are believed to be a product of cosmic–ray disintegration and may be electrically neutral or carry either a positive or negative charge. The principal types are the mu–meson and pi–meson. [<Gk. mesos middle]