How is 'interest' best defined?

Interest is best defined as the cost of borrowing money or the return on an investment. When a borrower takes out a loan, interest is the fee charged by the lender for the privilege of using their money. This fee is typically expressed as a percentage of the principal amount borrowed and is paid over the life of the loan. Conversely, when someone invests money, interest represents the earnings received from that investment, reflecting the time value of money. This definition encompasses both the lending side, where it is considered an expense, and the investing side, where it is viewed as income generated from assets. Understanding interest in this dual context is essential for effective financial management, as it plays a critical role in determining the overall cost of loans and the profitability of investments.