Full Definition
An ITIN (Individual Taxpayer Identification Number) is a 9-digit number issued by the U.S. Internal Revenue Service to individuals who are required to file federal tax returns but don't qualify for a Social Security Number. The format looks like a SSN (9XX-XX-XXXX) but always starts with 9.
Common ITIN holders: undocumented immigrants paying U.S. taxes, foreign nationals investing in U.S. real estate, dependents of U.S. citizens or residents who aren't eligible for SSNs, and some non-resident foreign professionals.
For apartment rentals, an ITIN allows the apartment community to run a tax-record-based background check rather than a Social Security based one. Some communities accept this; others require SSN.