Principal

Principal is the core amount of money in any lending or investing relationship — the sum originally borrowed, or originally invested — as distinct from the interest that grows on top of it.

On the borrowing side, your loan’s principal is what you actually owe. Interest is rent charged on that principal: every month, the lender computes interest as remaining principal × periodic rate. This is why “paying down principal” is the only progress that counts — interest payments keep the account current but shrink nothing. When a loan offers to apply extra payments “to principal,” it means the payment directly reduces the balance that generates future interest, which is what makes prepayment powerful (see amortization).

On the savings side, principal is your contributed money, and the distinction matters for understanding growth: after 30 years of compound interest, a portfolio’s balance is typically mostly earnings, not principal. In a Roth IRA specifically, the principal/earnings distinction has legal force — contributed principal can be withdrawn anytime tax-free, while earnings have rules.

One warning sign worth knowing: any loan whose balance grows despite payments (negative amortization) means payments aren’t covering interest, and principal is quietly increasing.