Critical illness insurance or critical illness cover is an insurance product, where the insurer is contracted to typically make a lump sum cash payment if the policyholder is diagnosed with one of the critical illnesses listed in the insurance policy.

The policy may also be structured to pay out regular income and the payout may also be on the policyholder undergoing a surgical procedure, for example, having a heart bypass operation.

The policy may require the policyholder to survive a minimum number of days (the survival period) from when the illness was first diagnosed. The survival period used varies from company to company, however, 28 days and 30 days are the most common survival periods used.

The contract terms contain specific rules that define when a diagnosis of a critical illness is considered valid. It may state that the diagnosis need be made by a physician who specialises in that illness or condition, or it may name specific tests, e.g. EKG changes of a myocardial infarction, that confirm the diagnosis.

In some markets, however, the definition of a claim for many of the diseases and conditions have become standardised, thus all insurers would use the same claims definition. The standardisation of the claims definitions may serve many purposes including increased clarity of cover for policyholders and greater comparability of policies from different life offices. For example, in the UK the Association of British Insurers (ABI) has issued a Statement of Best Practise which includes a number of standard definitions for common critical illnesses.    

Health insurance like other forms of insurance is a form of collectivism by means of which people collectively pool their risk, in this case the risk of incurring medical expenses. It is sometimes used more broadly to include insurance covering disability or long-term nursing or custodial care needs. It may be provided through a government-sponsored social insurance program, or from private insurance companies. It may be purchased on a group basis (e.g., by a firm to cover its employees) or purchased by individual consumers. In each case, the covered groups or individuals pay premiums or taxes to help protect themselves from high or unexpected healthcare expenses. Similar benefits paying for medical expenses may also be provided through social welfare programs funded by the government.

By estimating the overall risk of healthcare expenses, a routine finance structure (such as a monthly premium or annual tax) can be developed, ensuring that money is available to pay for the healthcare benefits specified in the insurance agreement. The benefit is administered by a central organization such as a government agency, private business, or not-for-profit entity.