A bond that sells at a significant discount (usually 20% or more) from face...

A bond issued with a very low coupon or no coupon and selling at a price far below par value. When the bond has no coupon, it's called a zero coupon bond.

It is a debt instrument issued with a very low coupon. They sell at substantial discount of their par value and are therefore referred to as deep discount bonds. A case of deep discount bond with no coupon called a zero coupon bond.