A method of securities distribution/underwriting in which the securities firm agrees to sell as much of the offering as possible and return any unsold shares to the issuer. As opposed to a guaranteed or fixed-price sale or bought deal, in which the underwriter agrees to sell a specific number of shares (and holds any unsold shares in its own account if necessary).
An agreement between an underwriter and an issuer in which the underwriter agrees to place as much of an offering with investors as possible, but is not responsible for any portion of the offering it fails to sell. For example, suppose an issuer makes a new issue of 100,000 shares. The issuer may make a best effort basis agreement with an underwriting firm for the underwriter to sell those shares to investors. If the underwriting firm only sells 90,000, however, it is not required to buy the remaining 10,000 from the issuer. This reduces the risk to the underwriter; to reduce the risk to the issuer, most best efforts are all-or-none offerings.