Payment made at a fixed interval.
A common example is the payment received by retirees from their pension plan. There are two main classes of annuities: annuities certain and contingent annuities. Under an annuity certain, a specified number of payments are made, after which the annuity stops. With a contingent annuity, each payment depends on the continuance of a given status; for example, a life annuity continues only as long as the recipient survives. Contingent annuities such as pension plans or life insurance depend on shared risk. Everyone pays in a fixed amount until the annuity begins; some will not live long enough to receive back all the money they have paid, while others will live long enough to collect more than they have paid.