interest
n. uncountablen. extra money you pay when you borrow money, or extra money you earn when you keep it in a bank. It is usually a small percentage of the total amount.
n. a charge for borrowed money, typically a percentage of the amount; or the return on investment for capital deposited in a financial institution.
The bank pays me interest on my savings every month.
High interest rates make it more expensive for people to take out a mortgage for a new home.
Compound interest allows an initial investment to grow exponentially over time, as the percentage is calculated on both the principal and the accumulated gains from previous periods.
From Middle English interest, from Old French interesse and interest (French intérêt), from Medieval Latin interesse, from Latin interesse.
Uncountable when referring to the financial concept or rate; often used with the preposition 'on' ('interest on a loan').
I paid many interests on the loanI paid a lot of interest on the loanIn its financial sense, interest is uncountable and does not take a plural form.