particle found in almost every atomic nucleus (the tiny spec of matter at the heart of an atom). Only the hydrogen nucleus has no neutron. All other atoms have one or more.
Like the proton and other baryons, the neutron consists of three quarks.
A free neutron (one not contained in an atomic nucleus) is radioactive. It undergoes beta decay to form a proton, an electron, and an electron-antineutrino. This what happened to most of the neutrons created by the big bang.
Free neutrons easily pass through atoms, because they have no electrical charge, and so they form highly penetrating radiation, interacting with matter only through collisions with atomic nuclei. These do not happen very often.
The neutron is important in forming a chain reaction in nuclear fission, as used in nuclear reactors and atom bombs. The absorption of neutrons by other nuclei, exposed to the high neutron densities in nuclear reactors, generates radioactive isotopes useful for a wide variety of purpose.
The neutron was discovered in 1932 by the English physicist James Chadwick.