Its not a monopoly over all ecommerce. However there are certain markets where the sellers, for example book publishers, are totally at the mercy of amazon because they have become the only de facto distribution channel for certain products like e-books (there are others as well). So its great for customers but it still has severe negative consequences for authors and publishers. Technically this is known as a "monopsony" but it's bad just in the same way a monopoly is.

So I guess any company becomes a "monopoly" if you reduce enough the definition of its field?!

The key thing economists look to, to define a monopoly, is that the company has the power to set prices. This is in the economic sense: that there isn’t some give-and-take supply-and-demand setting prices for them, and they just happen to record that.

When Office Depot was considering a Staples merger a while back, the Feds looked at the price of corporate office supply services in markets where there was only one of each firm, and found them higher than in markets where both competed. This despite the fact they didn’t have a monopoly on, say, copy paper.

Just because Amazon shoppers may ignore prices elsewhere doesn't mean Amazon can "set" prices.

It may mean Amazon understands their shoppers' price sensitivity, to be sure, but I can find almost anything on there elsewhere for less, or also for more.

Indeed.

I don't know about Target and Walmart as on-line entities, but I do know that there is limited overlap between the merchandise you find on eBay and the merchandise you find on AliExpress. So it is often (usually?) not true that if you can buy an item on website X you can also buy it at site Y.

You don’t need to be a monopoly to violate antitrust law.

Amazon fits every definition of a monopoly considering they have >50% market share, tyrannical control of the supply-chain in which they can flex their muscles with no repercussions and life-and-death control over sellers.