This doesn't clear anything up though. Who gets to define the boundary/scope of the market being measured? It's pretty easy to manipulate this to make it seem like a company has a "dominant" share even if that's not the case from the consumers point of view.

Take the anti-trust case against Microsoft, for example, the market was defined as that for computer operating systems for stand-alone personal computers using microchips of the kind manufactured by Intel. This left out not only operating systems running on Apple computers but also other operating systems such as those produced by Sun Microsystems for multiple computers or the Linux system for stand-alone computers. In its narrowly defined market, Microsoft clearly had a “dominant” share.

Also lots of people use Amazon to search for products so it's not clear to me how there is a market failure here.

If your point is that "monopoly" has a different meaning in a colloquial/political context and in a legal context, that it relies on non-obvious definitions of what the relevant market, what dominant is, and that it varies by country, I do not think it strongly supports the assertion that it does not have a loose definition.

I was mainly arguing against

> "Monopoly" is one of such words (others candidate include: freedom, democracy, justice).

-- there is a much more rigorous definition of Monopoly than the other ones you listed. You're claiming that it is some undefined concept that varies from person to person, but I think that's only true in the sense that "encryption" or "space-time" has a loose definition; most lay people don't understand technical jargon, and often misuse the jargon when they think they know what they are talking about. But that doesn't mean there _isn't_ a well-defined meaning that is agreed upon by the specialists that actually work in the domain in question.

The other words you listed do not have clear-cut legal definitions in the same way; they are not technical jargon defining criteria for enforceable regulations with tests and case-law backing them up.

Fair point.