Callable Bond

A callable bond lets the issuer repay early, typically when rates have fallen and it can refinance more cheaply. You have effectively sold the issuer an option, so you're paid a higher yield in exchange for the worst kind of asymmetry: you get called away exactly when your bond has become valuable. This caps upside and creates negative convexity, which is why callable bonds are quoted on yield-to-worst rather than yield-to-maturity.