I'm going from memory, but I have looked into the actual definition of a monopoly in the context of anti-trust. If I recall correctly, it doesn't even explicitl

I'm going from memory, but I have looked into the actual definition of a monopoly in the context of anti-trust. If I recall correctly, it doesn't even explicitly say that you need a market majority. It basically says that if you have undue market influence and power and use it then you have a monopoly. A market majority is simply one of the most common ways that that is attained. Again, that is from memory. I'd have to search for the language again to be sure. Like you point out though with it being tested in court, maybe precedence is strong. Those laws and definitions were written a long time ago. Today's companies like Google, Apple, Facebook, and Amazon are very different from companies of yesteryear.