elementary particle

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Any subatomic particle, such as one of the fundamental fermions or gauge bosons, that may be a component of other particles but is not made of components itself.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun physics fundamental particle; any of the subatomic particles that does not consist of other, smaller particles; the gauge bosons, leptons and quarks

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun (physics) a particle that is less complex than an atom; regarded as constituents of all matter