

Megacorporation, mega-corporation, or megacorp, a term popularized by William Gibson, derives from the combination of the prefix mega- with the word corporation. It has become widespread in cyberpunk literature. It refers to a corporation (normally fictional) that is a massive conglomerate (usually private), holding monopolistic or near-monopolistic control over multiple markets (thus exhibiting both a horizontal and a vertical monopoly). Megacorps are so powerful that they can ignore the law, possess their own heavily armed (often military-sized) private armies, be the operator of a privatized police force, hold "sovereign" territory, and even act as outright governments. They often exercise a large degree of control over their employees, taking the idea of "corporate culture" to an extreme. Such organizations as a staple of science fiction long predate cyberpunk, appearing in the works of writers such as Philip K. Dick (Do Androids Dream of Electric Sheep?, 1968), Thea von Harbou (Metropolis, 1927), Robert A. Heinlein (Citizen of the Galaxy, 1957), Robert Asprin (The Cold Cash War, 1977), and Andre Norton (the Solar Queen novels). The explicit use of the term in the Traveller science fiction roleplaying game from 1977 predates Gibson's use of it.
If a corporation has a market capitalization of $1 trillion, it absolutely counts as a megacorp. Anything down to $100 billion qualifies, to be honest. However there's something of a cultural aspect to megacorps as well. A start-up that convinces backers to throw a trillion dollars at it is nowhere near as all-encompassing as something like Samsung, where its workers can literally be born in Samsung hospitals, be raised in Samsung-funded schools, be trained to use Samsung military equipment, and die in Samsung hospitals.
Still, this is ultimately about the transnational powers of corporations, especially tech corporations, so there's not going to be much distinction between a regular but overly-large corporation and a cyberpunk-style megacorporation. It's all the same level of beastly wealth disrupting society anyway!
