social security number

Short definition from Wiktionary

Noun

1. 1
   A nine-digit number issued to citizens, permanent residents, and temporary (working) residents by the Social Security Administration under section 205(c)(2) of the Social Security Act, codified as 42 U.S.C. § 405(c)(2), introduced primarily to track people for taxation purposes, and in recent years serving as a de facto national identification number.
2. 2
   A corresponding number in other countries.
3. 3
   Alternative form of social security number.