(type of lepton that is stable and has an electric charge of -1)
An electron is a type (i.e., flavor) of lepton
that has an electric charge of -1 and is stable.
(A lepton is an elementary particle that is a fermion,
i.e., with 1/2 integer spin, but unlike a quark,
does not respond to the strong force.)
Through the electromagnetic forces subject to quantum-mechanical
restrictions, electrons can be bound to an atomic nucleus forming
atoms and assist in binding atoms together into molecules.
Electrons can exist free from atoms and such free electrons can make up
some of the particles of a plasma (and most baryonic matter
is in the form of plasma and most electrons in the universe are free of atoms).
An electron is very light compared to protons and neutrons,
on the order of 1/1800 of their mass, but the energy-equivalent
of its mass (its rest energy) is considerable,
0.511 MeV, tens of thousands of times the energy
needed to ionize an atom.
Electrons are created by beta decay
and by pair production, the latter which
produces both an electron and a positron
(the antimatter equivalent of an electron),
requiring twice the rest energy of an electron.
The term photoelectron refers to an electron freed by the
energy of a photon, from a molecule (this action being termed the
photoelectric effect) or from an atom (termed photoionization).