elementary particle - definition

Definition: A particle believed to be incapable of subdivision. The term fundamental particle is now more generally used. The status of "elementary particle" can change as the result of experiments or theories, progresively getting smaller as higher and higher energies (equivalently: smaller and smaller size scales) are explored by the particle accelerators around the world.

Examples: Until about the end of the 19th Century, the fundamental particle was generally thought to be the atom. Later it was observed that the atom itself had parts - the electrons, protons and neutrons. In the middle of the 20th century, it was observed that the protons and neutrons are not fundamental, being made up of quarks. The status of quarks as elementary remains under some debate.