A single issue of a bond made in two or more tranches. For example, a company may announce that is intends to issue $10,000,000 in bonds in two tranches of $5,000,000. Series bonds are common to collateralized mortgage obligations, which are backed by pools of mortgages. These mortgages are arranged in tranches that mature at different times, for instance, in 10 years, 15 years, and 30 years. See also: Serial bond.