Dirty price (also called 'full price' or 'invoice price') is what you actually wire to the seller when you buy a bond. It equals the clean price (the quoted market price) plus accrued interest — the interest that has accumulated since the last coupon payment. Bonds are quoted using clean price so comparisons aren't distorted by coupon payment timing, but you always pay dirty price at settlement. For example, if a bond quotes at 98 (clean) with 0.5 in accrued interest, you pay 98.5 (dirty). The name 'dirty' is market slang reflecting that it includes the 'messy' accrued interest component.