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). Over time, the number has been used for more diverse purposes than simply taxes, essentially making it a de facto national identification number.[2]http://en.wikipedia.org/wiki/Social_Security_number