[signal11]

That depends on national laws. In the UK, for example, > 25% market share makes you a monopoly.

[mcintyre1994]

Is that if no competitor also has 25%? Android and iOS must both be above 25% in the UK, which of those are considered a monopoly?

[lambda]

Don't think about the laws in terms of a single monopoly. The laws should be more accurately described as anti-trust or competition laws.

A handful of large players, each with less than 50% market share, can also distort the market and engage in anti-competitive tactics.

https://en.wikipedia.org/wiki/Competition_law