DEFINITION of 'W-9 Form'. An IRS form, also known as Request for Taxpayer Identification Number and Certification, which is used by an individual defined as a U.S. person or a resident alien to verify his or her taxpayer identification number (TIN).f you are defined as a foreign person for federal tax purposes, you do not use the W-9 form. Rather, the IRS requires you to use the appropriate W-8 form. Even though employees are legally required to supply certain personal information to their employers, an employee's privacy is protected by law.