Accrual Period
The time interval over which bond interest accumulates — typically from one coupon date to the next.
The accrual period is the window during which interest 'accrues' on a bond. For a bond paying semi-annual coupons on June 1 and December 1, the accrual period from June 1 to December 1 is 183 days (or 6 months, depending on day count convention). If you buy the bond on September 1 (3 months after the last coupon), you owe the seller accrued interest for those 3 months. Think of it as rent for the bond: every day you hold the bond, you earn a little interest. The accrual period resets on each coupon date. Day count conventions (30/360, Actual/365, Actual/Actual) determine exactly how many days count in each period, which affects accrued calculations.