An Individual Taxpayer Identification Number (ITIN) is a nine-digit tax processing number issued by the IRS for individuals required to file a federal tax return but ineligible for a Social Security Number (SSN). The application process involves filing Form W-7 and providing supporting documents; however, ITINs do not authorize work in the U.S. and are solely for tax purposes. It's crucial for applicants to adhere to IRS guidelines for efficient processing and renewal of their ITINs.