subatomic particle

n. elementary particle, individual elemental particle that compose the whole atom

A subatomic particle is an elementary or composite particle smaller than an atom. Particle physics and nuclear physics are concerned with the study of these particles, their interactions, and non-atomic matter composed from them.Subatomic particles include the atomic constituents electrons, protons, and neutrons. Protons and neutrons are composite particles, consisting of quarks. A proton contains two up quarks and one down quark, while a neutron consists of one up quark and two down quarks; the quarks are held together in the nucleus by gluons. There are six different types of quark in all ('up', 'down', 'bottom', 'top', 'strange', and 'charm'), as well as other particles including photons and neutrinos which are produced copiously in the sun. Most of the particles that have been discovered are encountered in cosmic rays interacting with matter and are produced by scattering processes in particle accelerators. There are dozens of subatomic particles.
See more at Wikipedia.org...

Noun
1. a particle that is less complex than an atom; regarded as constituents of all matter
(synonym) elementary particle, fundamental particle
(hypernym) particle
(hyponym) antilepton
(part-holonym) atom