My motivation for the question is: prior to the Age of the Web most product and service categories had several more-or-less-equal competitors: if you wanted to fly from NYC to LA there were several major airlines to choose from. Buying (or renting) a car presented a bunch of options, as did fast food restaurants, insurance companies, cameras, TV's refrigerators and other white goods, hotels, and almost everything else. It was rare for a company to sustain more than 50% market share.
But nowadays there are a whole bunch of product or service categories where there's one 900 pound gorilla and whoever is number two is way back in the dust somewhere. Google has 79.88% of all searches (Statcounter, August 2016) Amazon dominates online retail - I have no idea who #2 is. YouTube dominates video sharing; Facebook dominates social media with 1.2 billion daily active users. Windows dominates desktop operating systems, etc etc. And IDC reported this week that Android is now 85% of smartphones. Even in arcane categories like software development forums, there's StackOverflow and then who's number 2?
So what's a good, recognizable word for that kind of dominance, where one entity totally dominates its category, but is not technically a monopoly? Facebook, Android, Google, Windows, all have competition but whoever is number 2 is way back in the dust.