Fermion, FEHR mee on or FUR mee on, is any member of a certain class of atomic and subatomic particles. Every particle is classified as either a fermion or a boson. The simplest fermions are leptons and quarks. These are elementary particles--that is, they have no known smaller parts. The electron is one of six types of leptons. There are also six types of quarks. Fermions also include composite objects that are made up of an odd number of fermions. Neutrons and protons are such fermions. Each is composed of three quarks. Other composite fermions include certain nuclei, atoms, and molecules.