Gluon
Definition
The particle that glues quarks together — carriers of the strong force that themselves carry color charge.
Technical definition
Massless spin-1 gauge bosons of QCD, in eight color varieties. Unlike photons, gluons interact with each other, causing confinement and asymptotic freedom.
Example
Gluons supply roughly 99% of a proton's mass — the quarks themselves are light; the binding energy is the bulk of what you weigh.
Common confusion
Gluons can't be isolated the way photons can. Pull quarks apart and the gluon field simply snaps into new quark pairs — free gluons are never observed.