The key thing economists look to, to define a monopoly, is that the company has the power to set prices. This is in the economic sense: that there isn’t some give-and-take supply-and-demand setting prices for them, and they just happen to record that.
When Office Depot was considering a Staples merger a while back, the Feds looked at the price of corporate office supply services in markets where there was only one of each firm, and found them higher than in markets where both competed. This despite the fact they didn’t have a monopoly on, say, copy paper.
