Concentration of risk
\ˌkɑnsənˈtreɪʃən\ \ʌv\ \rɪsk\
This is a measure for how concentrated longevity risk is among individual members of a given population.
For example, a pension plan that includes one participant with a very large pension who makes up 50% of the liabilities would have a high concentration of risk. A pension plan consisting of a large number of participants, all making up a roughly equal proportion of the liabilities would have a low concentration of risk.