The bookrunner is the principal underwriter or lead coordinator of the issuance of new equity, debt or securities.
In investment banking, a bookrunner is the lead underwriting firm that maintains or is in charge of the books at the time of the issuance of new shares of the client firm.
The bookrunner acts as the lead underwriter and usually works with other investment banks to form an underwriter syndicate, thereby creating the initial share sales department.
In a leveraged buyout, the bookrunner represents one of the participating firms and works with other participating firms.