Senator Herb Kohl: But you do recognize that in the words that are used and antitrust kind of oversight, your market share constitutes monopoly, dominant -- special power dominant for a monopoly firm. You recognize you're in that area?

Eric Shmidt: I would agree, sir, that we’re in that area....I'm not a lawyer, but my understanding of monopoly findings is this is a judicial process.

From: https://www.businessinsider.com/is-google-a-monopoly-were-in...

Also, the FTC's initial memo from 2012 that somebody higher up in the food chain quashed is pretty interesting: http://graphics.wsj.com/google-ftc-report/

In short, dominant market share in web search. Though I think you could argue other things, like dominance in affordable smart phones. Android is effectively a monopoly for people that can't afford an iPhone.