Reinsurance is insurance
In law and economics, insurance is a form of risk management primarily used to hedge against the risk of a contingent, uncertain loss. Insurance is defined as the equitable transfer of the risk of a loss, from one entity to another, in exchange for payment. An insurer is a company selling the...

 that is purchased by an insurance company (insurer also sometimes called a "cedant" or "cedent") from another insurance company (reinsurer) as a means of risk management
Risk management
Risk management is the identification, assessment, and prioritization of risks followed by coordinated and economical application of resources to minimize, monitor, and control the probability and/or impact of unfortunate events or to maximize the realization of opportunities...

. The reinsurer and the insurer enter into a reinsurance agreement which details the conditions upon which the reinsurer would pay the insurer's losses (in terms of excess of loss or proportional to loss). The reinsurer is paid a reinsurance premium by the insurer, and the insurer issues insurance policies to its own policyholders.