A corporate bond that is lower in priority—i.e., subordinated—to other debt that the company has.
Big companies have lots of layers of debt, and they need to prioritize to figure out what gets paid first. Subordinated bonds are lower rated and they get paid after the higher-priority, higher-rated bonds.
P.S. Debentures are usually not secured by specific collateral, so they're the equivalent of a standing room ticket at an English soccer match.
Good luck.