Definition
Interest Rate
The percentage a lender charges to borrow, or a bank pays to save.
An interest rate is the price of money over time, expressed as a percentage. On debt, it is what you pay the lender; on savings, it is what the bank pays you.
Small differences add up enormously over time thanks to compounding. Shaving a few points off a loan's rate, or earning a few more on savings, can be worth thousands of dollars.