A period of time (usually six months) during which company insiders, including management, are not allowed to sell their shares (in the company itself). This period which follows an initial public offering (IPO) is usually determined by contractual agreement between and the underwriters. However, the lead underwriter has the option to end this period earlier. At the expiration of the lock-up period, employees and management of the IPO firm are free to sell their shares without restrictions.