Steam

Yes, this, so many people use words but don't know what they mean.

Yes, Steam is an effective monopoly.

You don't need to literally be the only possible option, the entire market, to be 'a monopoly'.

Economists very often refer to a company that has just a vastly oversized market share and other kinds of market influence, ws compared to the next comptetitor or set of competitors, as a monopoly.

Like uh, Walmart has a decent chance of having a local, effective monopoly on the grocery market, if you live in a whole lot of US cities.

And also yes, when it comes to anti trust law... yeah you generally have to do things that are either current anti competetive or anti consumer practices, of have done them in the past to acquire your monopoly status, to be broken up by a possible Sherman Act based action.

It is actually possible to become a monopolist without doing anything particularly uncommon in the market, or underhanded... its possible that just no one bothers to meaningfully compete with you untill its too late.