The amount of money you start with in interest calculations is called the principal.
When you invest money or take out a loan, the original sum of money put in or borrowed is referred to as the principal. This is the initial amount before any interest or other earnings are added to it or before any interest is deducted in the case of a loan. In the context of savings or investments, the principal is the amount that you initially place in an account to generate interest or investment earnings. In the context of a loan, the principal is the amount that you initially borrow and will pay interest on.
