Short answers, pulled from the story.
A proton is a stable subatomic particle with a positive electric charge of +1 elementary charge. Its mass is approximately 1,836 times that of an electron and slightly less than that of a neutron. One or more protons sit in the nucleus of every atom.
Ernest Rutherford named the proton in 1920, deriving the word from the Greek for "first." His 1919 experiments striking air with alpha particles from radium led him to conclude that hydrogen nuclei were a constituent part of the nitrogen nucleus, a result described as the discovery of protons.
A proton is a composite particle made of three valence quarks: two up quarks with charge +e each and one down quark with charge minus e. These are held together by the strong force, which is mediated by gluons, making the proton a baryon and a type of hadron.
The rest masses of a proton's quarks account for only about 1 percent of its total mass. The proton's mass runs about 80 to 100 times the sum of its three valence quarks' rest masses, with the rest coming from quantum chromodynamics binding energy, including the kinetic energy of the quarks and the energy of the gluon fields.
The spontaneous decay of a free proton has never been observed, so the Standard Model considers it stable. Some grand unified theories predict proton decay with lifetimes between 1031 and 1036 years, and experiments at the Super-Kamiokande detector in Japan have set lower limits for specific decay paths.
The number of protons in an atom's nucleus is its atomic number, represented by the symbol Z, and it defines which element the atom is. For example, chlorine has an atomic number of 17, so every chlorine atom has 17 protons.
The proton's charge radius, with a root mean square value of about 0.8 femtometers, measures differently depending on method. The value from electron-proton scattering differs from the value found via the Lamb shift in muonic hydrogen, and unexplained differences remain despite a third high-precision measurement agreeing most closely with the muonic hydrogen result.