In short, it's a "massive" company that has both horizontal and vertical monopolies (controls most of the aspects of production of a lot of different things,) is so powerful that it either controls or can ignore the law, and has sovereign territory and its own standing army.
For those who haven't checked the long version already, it turns out there was in fact one real life example. The Dutch East India Company, which was actually a subset of the East India Company that i was comparing to the East India Trading Company. The aspects of the Dutch East India Company that make it a megacorp are the same elements that made it a perfect model for the fictional version. It's kind of ironic that the term megacorp was popularized by the cyberpunk genre, but the only definite real world example existing hundreds of years before the genre.
Surprisingly, the only modern company to come close to being a megacorp, according to wikipedia anyways, is Disney. It turns out that Disney _is_ the government of the Reedy Creek Improvement District in Florida. All Disney needs now is a standing army...
I do kind of wonder about some of the oil companies though, given what i've heard about their actions in third world countries. Do they not count as megacrops because they don't have much of a horizontal monopoly? Or does hiring mercenaries to terrorize and kill people who get in your way not count as a standing army? Or do you only count as a megacrop when you can pull off that kind of thing in more developed countries?