Definitions for junk bond
Random House Webster's College Dictionary
a corporate bond with a low rating and a high yield, often involving high risk.
Origin of junk bond:
junk bond, high-yield bond(noun)
a (speculative) bond with a credit rating of BB or lower; issued for leveraged buyouts and other takeovers by companies with questionable credit
A bond (an instrument of debt) which is considered below "investment grade" due to a significant risk of default by the issuer. The interest rate is higher in order to compensate holders for that risk.