I'm not redifining the term, economists are. Here is an etymology from dictionary.com
monopoly : "exclusive control of a commodity or trade," 1530s, from L. monopolium, from Gk. monopolion "right of exclusive sale"...
The people who have started using "monopoly" to mean "significant market power" and the like were doing a bait-and-switch to try to further an anti-business academic or political agenda. And most of the arguments you hear using the word "monopoly" imply the older definition that I'm using, but apply it to much looser definitions. For example, MS was never even remotely close to having a monopoly on operating systems. In fact, such a thing is not even really fathomable.
PS: The reason you don't need 100% market share to count as a Monopoly is once your competition can't meet the full needs of the market so you can increase prices which in theory causes a loss of market share, significant barriers to entry can dramatically slow this process down.
I didn't quote your entire paragraph, but I'm referring to all of it.
This kind of reasoning (which is reasonable and extremely common) is totally rationalistic (which is properly defined, by the way, as being an argument that seems deductively valid but is divorced from the way reality really is). I could talk about this at length, but I'd rather just give a short example for now. Imagine I'm a big business. If I'm consuming a particular kind of product as an input to my production, and one particular supplier charges a very high price, it wlll be greatly to my interest to either invest capital to produce it on my own, or (much more likely) seek out new producers to enter the market. If the existing supplier then tries to manipulate prices to drive the new competition out, only planning to raise them right after, I'll just see through it and continue purchasing from the new producer. A group of companies in my industry could even come to a mutual agreement of this sort to ensure that new competition enters.
The point is, in a capitalist (that is, politically and economically laissez-faire system), people don't just keel over when there are problems in the market, if those problems are actually big enough to be worth solving.