MegaCorps, short for mega-corporations, are companies or organisations that have declared themselves independent of Factional authority, and have the power and clout to actually back that up. Typically this freedom comes from sheer scale of operations, unique or rare proprietary technology, or integration into the economy of the Factions to such a degree that suspension of services would be catastrophically disruptive.
The first MegaCorp is recognised to be the gargantuan Galactic Energy Supplies, which came together in the Ascendancy before they even encountered any other major spacefaring powers. The energy generation and storage technology developed by the early GES was a cheap, portable and efficient way of generating and storing power for the growing Terran domain, and GES was often involved or brought in to assist or devlop major energy projects. Meanwhile, the Noble Houses were too busy undermining each other's interests to prevent Galactic Energy Supplies from becoming utterly indispensable in all Terran colonial and military operations. When they began to build their own fleets of ships and move to gain freedom from Ascendancy laws, the most powerful Houses did try to move against them, but were quickly cowed when GES switched off their fusion generators and de-powered the entirety of the Trappist System for ten standard days.
All of the well-known MegaCorps maintain relations with each of the four Factions, and take pains to deal with each one in a generally fair and consistent fashion. This is part of what gives them their protection - angering a MegaCorp by taking large-scale action against their interests would most likely lead to a withdrawal of their services from the aggressors, and give the other Factions a substantial advantage against them. That is not to say that the Factions do not test the MegaCorps, and vice versa, usually in small ways and under the cover of deniability. “Pirate” raids, false-flag attacks, hostile takeovers, “unforeseen” disruption of services, infiltration and spying are all part of the delicate dance of power and intrigue between the MegaCorps and the Factions.
Joining a MegaCorp directly is usually a staged process, where an individual or company will come to the attention of the MegaCorp by completing outsourced work for them, and slowly building a good relationship with them. Due to the hazard presented by Faction infiltrators, MegaCorps only tend to directly hire new employees after they have worked with them for a considerable period of time, and even then only after substantial vetting and testing. A MegaCorp employee typically divests themselves of Factional ties, and swears allegiance to the MegaCorp, legally placing themselves under its authority rather than their own world or Faction's laws. By taking this step, the employee is admitted into the outermost circle of the Corp, and can begin to learn its secrets and work their way up its hierarchy.
The MegaCorps in operation today are important fixtures in the day-to-day life in the Orion Sphere. Most have survived for many hundreds or thousands of years, and have extensive holdings and operations across Known Space. They often act as brokers or fixers between the Factions, and their structure allows them to respond more quickly to opportunity or risk than the monolithic Faction authorities. Every MegaCorp that has survived to attain that status is unique. Their individual history and secrets are usually tied up in their specialist field of endeavour, and no two MegaCorps overlaps this speciality to a significant degree, or else they would not have risen to become MegaCorps in the first place.
Most major MegaCorps have substantial holdings across known space - their very status as a MegaCorp is dependant on dealing equally with all the Factions. The larger ones, such as Advanced Armaments and Galactic Energy Supplies have vast holdings, easily greater than multiple Ascendancy Great Houses combined. Most substantial inhabited worlds will have at least one facility controlled by each of the major MegaCorps, independent of local laws but typically performing a myriad of useful services for the world's inhabitants. Their operations on any given world might be limited to a single, embassy-like office, or vast networks of factories, power plants, mines, research labs or even spaceports.
Some MegaCorps also maintain extensive fleets of starships as well as their fixed holdings, and may even directly control entire planets or star systems in Faction-held space or beyond their borders. Their control and independence may be invested in soft power, contracts and webs of influence, or in the wilder reaches of space in the strength of their fleets, the influence of their networks of contacts or the might of their cash reserves. Smaller companies and starship crews are usually below their notice, but earning the favour or ire of a MegaCorp can create fantastic opportunities or terrible dangers for brave or foolhardy spacers.