"Supermajor" is a term, widespread since the late 1990s, used to describe the six largest wholly public oil companies.
The six "supermajor" oil companies are:
✧ BP (UK)
✧ Chevron (USA)
✧ ExxonMobil (USA)
✧ Royal Dutch Shell (Netherlands & UK)
✧ Total (France)
✧ ConocoPhillips (USA)
The supermajor oil companies came into being as the result of an extended period of low oil prices in the 1980s and 1990s which forced a massive consolidation of the oil industry through mega-mergers in order to wring extra profits out of massive economies of scale.
The supermajors are some of the largest and most powerful corporations on Earth. ExxonMobil is currently the largest corporation in the world, by market cap, and all six companies are in the top 25.
Naturally, the great wealth of these corporations has given them an outsized influence on politics, which they have generally used for evil. In particular, these companies, often pejoratively known as "Big Oil," have successfully pushed for reduced pollution standards, and all manner of government handouts and subsides, especially in the United States.
But as big and bad as these companies may seem, they actually don't control all that much of the world's oil - only a miniscule 6% of proven reserves. Most of the world's oil reserves are controlled by companies that are primarily state-owned. These true supermajors - we might call them "superdupermajors" - are the private piggy banks of dictators, demogogues and despots in places like Russia, Venezuela, China, and the Middle East, and have reserves the supermajors can only dream of.