What is the definition of A Junior Tranche?

A junior tranche is the lowest tranche of a security, i.e. the one deemed most risky. Any losses on the value of the security are absorbed by the junior tranche before any other tranche, but for accepting this risk the junior tranche pays the highest rate of interest.

Junior tranches are often known as mezzanine tranches. Of the total value of security the junior tranche rarely accounts for more than 10-20%.