In the United States, a Social Security number (SSN) is a nine-digit number issued to U.S. citizens, permanent residents, and temporary (working) residents under section 205(c)(2) of the Social Security Act, codified as 42 U.S.C. 405(c)(2).
The number is issued to an individual by the Social Security Administration, an independent agency of the United States government. Although its primary purpose is to track individuals for Social Security purposes, the Social Security number has become a de facto national identification number for taxation and other purposes.
A Social Security number may be obtained by applying on Form SS-5, Application for A Social Security Number Card.