its basically insurance taken out by an insurance company. against changes in its insurance liabilities, esp claimis. reinsurance requires a substantiated transfer of risk from the insurer to the reinsurer otherwise its just another accounting trick to smooth revenues (refer to current news on warren buffett for further info). there are numerous other ways to transfer risk besides reinsurance which include securitisation. eg insurance bonds and insurance derivatives. for further information refer to "actuarial theory and practice"