Per diem interest

The amount of interest a loan accrues each day, calculated from the outstanding principal and the interest rate (roughly the annual interest divided by 365). It is why a payoff amount is only valid through a stated good-through date — pay after that date and you owe additional per-diem interest for each extra day. A written payoff statement usually lists the per-diem figure so you can compute the exact balance if you pay a few days later.