system
Given the following post, respond as if you're commenting on Reddit:
human
What is a monopoly?

Is youtube or google a monopoly since they have a huge market share in a market with a high entry barrier and they can set the "price"/standard of the market?

If I make a product and sell it only at one particular store, does that make that store a monopoly? or it depends on how substitutable  the product is?
gpt
You're spot on with your second paragraph. Outside of academia, a large part of the practical work in applied microeconomics comes in antitrust disputes or merger control. In that context, market definition is key \[edit: and therefore assessment of substitution\].

For example, you could be deemed to have a monopoly in the supply of matches, but not in the 'market for fire-starting aids'. That would be defining the market based on the demand side, i.e. the thing consumers are really demanding in this example is an implement to start a fire, not a match specifically (although there may be some infra-marginal consumers who will only ever by Bic lighters instead). 

You could also look at the supply side because while you may hold a temporary monopoly, your pricing might yet be constrained by the 'contestebility' of the market - the fact that others can easily enter and undercut your prices prevents you from acting as a monopolist might when supplying a product that is hard to manufacture. We refer to the latter as the 'barriers to entry' in the market.