I don't think that's the definition of a monopoly that is problematic here, more the definition of a market, which is a more abstract group, composed of competi

I don't think that's the definition of a monopoly that is problematic here, more the definition of a market, which is a more abstract group, composed of competitors and that companies are supposed to be able to enter somehow easily. The debate there is whether online shops and physical shops are in the same market, and what geographical divisions make sense for online shops. It is also in the definition of tech markets, which are starting to clear up. We know for instance that it is not necessarily easy for a tech giant in ads/search (Google) to enter the social media arena, so these two things are separate. Hosting and cloud computing seem to be very similar markets, most players there having offers in both. I think the market of online banking is being defined right now with many unconventional players showing that ecommerce may actually belong to the same market as paypal. Defining monopolies is much easier, if you start from the (rather political) use of the concept : deciding when a company has to be broken up to preserve competition. MArkets are just a tool to create definitions of monopolies. That debate is too often obscured by various ideologies on the wisdom of the crowds and invisible hands imbued with various mystical properties.