A coupon is the interest that a bond pays to its holder at regular intervals, and the term is covered in full under coupon payment. The word comes from the era of paper bond certificates, which carried a sheet of small detachable slips. On each interest date the holder cut off, or clipped, one slip and presented it to a bank to collect the money. Bonds are now held electronically and interest is paid automatically, but the name has stayed.
In everyday market use the word has two meanings. It can mean the cash amount paid on each date, and it can mean the annual rate of interest stated on the bond, more exactly called the coupon rate. A trader who says a bond "has a 5 percent coupon" means that it pays 5 percent of its par value a year, often in two instalments. The coupon is fixed when most bonds are issued and should not be confused with the yield, which changes with the bond's price.
A bond with no interest payments is a zero-coupon bond. Interest earned since the last coupon date is the difference between a bond's clean price and the amount actually paid for it.