An agreement by stockholders not to sell their shares of stock for a specified period of time. Underwriters often require officers, directors, and principal stockholders of a company to agree to lockup all, or a portion of, their shares for a designated period after the public offering. Selling Stockholders may also be required to lockup their shares for an agreed upon period. A lockup may be absolute (i.e., for a specified period of time with no exceptions) or subject to the underwriter's discretion. When it is subject to the underwriter's discretion, the lockup may be released by the underwriter earlier than the agreed upon expiration date