Accrued Interest
Also called: accrued coupon, interest accrual
Accrued interest is coupon that has built up since the last payment date but hasn't been paid yet. When you buy a bond between coupon dates you pay the seller for it, because they earned it — otherwise buying the day before a coupon would be free money. It's calculated with a day-count convention that varies by market, and it's the difference between a bond's quoted clean price and the dirty price you actually settle.