Part of it depends on how you define things. They're not a monopoly in terms of having eliminated all their competitors, but they're a defacto monopoly in terms of being able to do the things a monopoly can. As an example, they can dictate pricing for the whole market as their margins are better than AMD's, so if AMD undercut them, they can retaliate by dropping their prices to the point AMD would have to sell at cost, so AMD can only sell things in the narrow price window where Nvidia doesn't feel threatened. On the other hand, AMD does exist and does sell things.