Elementary Particle
A particle with no internal structure, which makes up all the other composite particles: it is indivisible, its properties do not reflect the existence of any simpler object that it might contain. The "elementary" status of a particle can be questioned at any time, as it depends on the precision of the experiments. As science progressed, the atom, then the nucleus, then the protons/neutrons were first considered as elementary. Today, quarks seem to be. But for how long ?