(type of baryon that is stable and has an electric charge of +1)
A proton is a type of baryon
that has an electric charge of +1 and is stable.
(A baryon is a hadron, i.e., a composite particle,
that is made up of three quarks.)
Protons are subject to the strong force, the force
which draws together protons and neutrons to form an atomic nucleus
as well as holds the three quarks of a neutron or proton together.
The nucleus of all elements include protons, but
the simplest/most-common hydrogen nucleus consists of
just a proton, and ionized hydrogen is just free protons.
At room-temperature velocities, protons are merely hydrogen and
not dangerous, but higher-velocity protons found in the solar wind
and cosmic rays are a danger to humans and to equipment in outer space.
Neutrons and protons have roughly the same mass
(around 940 MeV), which is about
1800 times that of an electron (around 0.5 MeV).
Protons were created very soon after the Big Bang when the
temperature dropped sufficiently that quarks combined into
protons and neutrons. Radioactive decay sometimes converts a
proton within a nucleus to a neutron or vice versa.