An underwriting is a contract between a brokerage firm and an issuer through which the former commits to buying a security issue, or part of it, to later resell it to investors. It can be either a firm commitment offering of a best-efforts offering.

Firm commitment offering: contract through which the brokerage firm commits to buying from the issuer a part or the entirety of an issue at a predetermined price, with the purpose of reselling it to investors.

Best-efforts offering: contract in which the brokerage firm makes a commitment to the issuer to do its best to sell the offering, but does not guarantee the sale nor assume any risk regarding securities that are not sold.