An Individual Taxpayer Identification Number (ITIN) is a unique number issued by the U.S. Internal Revenue Service (IRS). This number is issued to those individuals who are not eligible to obtain a Social Security Number (SSN), but who need identification to file their taxes.
ITINs are issued to individuals who have tax obligations to the IRS, regardless of whether they are residents or nonresidents of the United States. This includes foreign nationals living in the U.S., foreign nationals who earn income from U.S. sources even though they do not live in the U.S., as well as certain dependents and spouses who are not eligible for a Social Security Number.
Having an ITIN offers several benefits, such as tax compliance, eligibility for tax credits, the ability to establish a credit history, and more, as detailed here.
ITINs consist of nine digits and always begin with the number 9. It is important to note that these numbers do not grant the right to work in the U.S., nor do they indicate immigration status.
In short, an ITIN is an identification number issued by the U.S. IRS for those who cannot obtain a Social Security Number, but require an identification number for tax purposes.