Usury is lending money and charging interest that is far too high — money-lending that squeezes the borrower. It is an uncountable noun: you say 'usury', never 'a usury' or 'usuries'. The related words are 'usurer' (the person who lends this way) and 'usurious' (an adjective for such rates or such a loan).

Today the word is mostly legal or moral. Many countries have usury laws, which set a legal ceiling on interest rates; a loan above that ceiling is illegal. When a newspaper writes that a lender was charging 300% a year, that is when 'usury' appears. It is a heavy word — using it means you think the lender was doing something wrong, not merely expensive.

Though actually, the word used to mean something much wider, and this is where students get confused reading older books. For centuries in Europe, and in Islamic and Jewish law too, usury meant charging ANY interest at all, however small. Lending at interest was treated as a sin, which is why the money-lenders in old stories are so often hated figures, and why the Church argued about it for hundreds of years. So in a book from 1400 'usury' probably means all interest; in a newspaper from today it means only unfairly high interest. The meaning narrowed as banking became normal.

That older meaning has not disappeared. In discussions of Islamic finance, English writers still use 'usury' to translate 'riba', where it keeps the strict old sense of any interest — which is why Islamic banks are built to work without it.