Quark
A quark is an elementary particle which makes up hadrons, the most stable of which are protons and neutrons. Atoms are made of protons, neutrons and electrons. It was once thought that all three of those were fundamental particles, which cannot be broken up into anything smaller, but after the invention of the particle accelerator, it was discovered that electrons are fundamental particles, but neutrons and protons are not. Neutrons and protons are made up of quarks, which are held together by gluons.