A Taxpayer Identification Number (TIN) is a nine-digit number that is used to identify individuals and businesses for tax filing and reporting purposes. It is issued by the Internal Revenue Service (IRS) and is also known as a Federal Tax Identification Number. The most common TIN is the Individual Taxpayer Identification Number (ITIN) assigned to a person who is required to file taxes but does not have a U.S. Social Security number.

The TIN is used to process and monitor tax records, track and organize tax payments, identify taxpayers for tax law enforcement and ensure that taxpayers are paying the correct amount of taxes. Each TIN is unique and can be assigned to individuals and entities at any time.

Employer Identification Numbers (EIN) and Adoption Tax Identification Numbers (ATIN) are two other types of TINs. An EIN is assigned to employers, sole proprietors, corporations, partnerships, and other business entities. An ATIN is used to identify adoptees when the adopting parent's taxpayer identification number is not available or disclosed.

It's important to note that a TIN is not the same thing as a Social Security Number. An individual TIN is used when filing taxes as an individual, whereas a Social Security Number is used to identify a person for employment, retirement, and Social Security benefits when they are earned.

To obtain an ITIN, you have to complete Form W-7, file it with the IRS along with any documentation needed for a tax filing, and wait for the IRS to issue an Individual Taxpayer Identification Number. In some cases, individuals may be able to obtain a TIN from their employer or from the Social Security Administration.

It’s important for taxpayers to protect their TIN—just like they would with a Social Security Number. Individuals should only give their TIN to the IRS, employers, financial institutions, and other individuals or entities when needed for tax-related activities. Illegally using someone else’s TIN can result in criminal charges.