if you create a product thats unique or defines a category you start by having a market share of 100 percent. Market share does not define monopolies. What defines monopolies is the absence of alternatives. Like you know, the postal service to send letters. funnily nobody talks about those.
The economic definition of monopoly is 'price setter' as opposed to 'price taker'. The government actually defines that as 'market power'. But in order for it to be something 'anti trust' etc. then it's more complicated.
Sherman Act, Section 2 [1] - definitely worth a read.
