Divestiture is a political term used to describe the breaking up of major oil companies.
Divestiture can take two forms: horizontal and vertical. Horizontal divestiture would force big oil companies to shed their non-oil business, like chemicals, coal, and nuclear energy. Vertical divestiture would require the oil companies to divide their operations by function (exploration, production, refining, transportation, marketing), and spin off all but one operation.
Divestiture, today, has generally come to mean that integrated oil companies would no longer be allowed to own and operate retail marketing facilities.
See also Divorcement.