Definition of 'Flat Bond'
A debt instrument that is sold or traded without accrued interest, the fraction of the bond's coupon payment that the holder earns between periods of bond payments. There are three possible reasons that a bond would not have any accrued interest:
Investopedia explains 'Flat Bond'
If a bond price is referred to as "dirty" it means that the interest accrued since the last coupon payment is included in the price of the bond. If it is referred to as "clean," the price does not include any accrued interest.
Corporate and municipal bond issuers assume a 30-day month and a 360-day calendar to calculate the accrued interest on a bond. However the accrued interest on government bonds is usually determined on the basis of the actual calendar day from date of issuance (called the actual/actual day count).