I. noun
: any of the subatomic units of matter and energy (as the electron, pion, proton, and photon) formerly held to be indivisible entities that are distinguished by a characteristic mass and by quantum properties (as charge and spin)
II. noun
: one of the structural units of mitochondrial cristae that are observed by the electron microscope usually as spheres or stalked spheres and that are prob. sites of fundamental energy-producing reactions