What is APR?

The term annual percentage rate of charge (APR), corresponding sometimes to a nominal APR and sometimes to an effective APR (EAPR), is the interest rate for a whole year (annualized), rather than just a monthly fee/rate, as applied on a loan, mortgage loan, credit card, etc. It is a finance charge expressed as an annual rate.

Those terms have formal, legal definitions in some countries or legal jurisdictions, but in the United States:
(1) The nominal APR is the simple-interest rate (for a year).
(2) The effective APR is the fee+compound interest rate (calculated across a year).