That depends on jurisdiction.

In the EU, it is "dominant market position" (and explicitly so) that's the threshold for the Commission to take competition action, for instance.

would AWS be considered "dominant" though? they are the indisputably the market leader, but there's plenty of competition

(not trying to argue they aren't dominant. just curious)

I don't see why not.

British Airways only had a 38% market share when they were sued for abusing their dominant market position in 1998 (which was upheld by the Court of Justice in 2007)

Presumably they could. I mean, if the EU legislators had intended for a monopoly to be necessary to be considered “dominant”, then they could have written “monopoly”, couldn't they? They didn't write that, so it seems safe to presume they didn't intend that. And if anyone is dominant, then who, if not the market leader?

> Leveraging your monopoly to compete in another area is illegal. Leveraging a strong position isn’t and Amazon is a long ways from a monopoly.

That really depends on interpretation, which has shifted over time and continues to shift. IIRC, recent interpretations of some types of anti-competitive behavior have been rather literal and required something very close to a literal monopoly, which has had the effect of neutering antitrust law in all but the most blatant of cases.

My understanding is that it's arguable that it's anti-competitive to leverage market share advantages more broadly (e.g. antitrust law could be used to constrain/break-up a duopoly).

Amazon, Google, Microsoft is not a duopoly :)

And while there are probably some services on AWS you can’t find at a competitor - that’s mostly a choice (aka profit margin) made by the competitor.