A type of insurance. Under-writers for a stock or bond issue have a backstop in place to make sure that the security issue will be purchased. To do so, the underwriters assemble a group of sub-underwriters, often institutional investors who are prepared to take a certain share of the offering that remains unsold. This type of underwriting deal may be called a backstopped deal or a firm-commitment underwriting deal. Backstop also refers to a government agency or loan guarantee program that will insure a companyÂ’s debt or its credit line.