I was simplifying it for OP.
I'm also not a lawyer.

but, steam does not fit the definition of a monopoly under US law, as that would require them to have “restricted commerce or trade” to achieve their dominant market position.

The word for a market dominated by only a few very large players is oligopoly, not... polyopoly.

Not saying you're saying that, just saying.

...

As to the etymology...

Its derives from Greek.

A monopoly has one (mono) influential seller for many (poly) consumers.

An oligopoly has a few, wealthy (oligo, as in oligarch, oligarchy) sellers for many (poly) consumers.

Importantly, in Greek, poly is closely related to polis, meaning basically 'all of the people/citizens'.

This is also where English gets 'Politics' from.

...

Also, I wrote a whole other comment, but the mere existence of any competitors, no matter how small... doesn't mean you aren't a monopoly.

Its just means you aren't a perfect monopoly, which basically never exists in real life, outside of public utilities.

If the rubric for 'is it a monopoly?' was 'do any competitors exist?', then basically no company that's ever been broken up or regulated for being a monopoly was actually a monopoly.

Because that's the way i decided to dumb it down. Apparently it wasn't dumbed down enough.