What is the meaning of INCURRED BUT NOT REPORTED (IBNR)?

In insurance, losses occurring over a specified period that have not been reported to the insurer. IBNR losses are often calculated as a percentage of claims paid and claims outstanding and are reported in an insurer's annual report. Reinsurers establish IBNR reserves as a part of their rating plans under a facultative reinsurance treaty, lest an overly optimistic view of treaty results lead to further under-rating on a book of business. Example: Product liability losses are seldom reported during a policy year. This "tail" of claims will upset any rating plan, unless an IBNR reserve is established and factored into the profit picture.