Monopoly
Etymology
From Greek monopōlion, "right of exclusive sale," from monos, "single, alone," plus pōlein, "to sell." The word arrived in English in the 1500s, already meaning an exclusive grip on selling something.
noun · /muh-NOP-uh-lee/
A market in which a single seller controls a product or service, with no real competition.
What it names
It explains why a lone provider can charge more and improve less: with nowhere else to go, customers have lost their strongest bargaining tool.
Example
The only pharmacy for fifty miles had a local monopoly and priced its medicines accordingly.
Related idea
Its mirror image is monopsony, where there is a single buyer rather than a single seller.