A parton is a constituent resolved inside a rapidly moving hadron by a hard scattering process. In quantum chromodynamics, the relevant partons are quarks, antiquarks and gluons. A parton distribution function describes the momentum-fraction distribution of a specified constituent at a stated resolution scale.
In deep inelastic scattering, a high-energy probe transfers enough momentum to resolve structure much smaller than the hadron. The parton model approximates the short collision as scattering from one constituent, with the remaining structure entering through its distribution. Which partons are resolved depends on the scale; increasing the resolution permits additional radiation and pair production. A parton is therefore not a permanently labeled classical bead inside the hadron, and the distributions are not probabilities for finding isolated free quarks.
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