What is the term for the nine-digit number used for identification purposes?

The term for the nine-digit number used for identification purposes is the Social Security Number (SSN). The SSN is a unique identifier issued by the Social Security Administration in the United States primarily for tracking individuals' earnings and benefits within the Social Security system. However, it has become widely used beyond its original purpose as a means of identification for various processes, such as applying for loans, credit cards, or governmental services.

The other options refer to different types of identification: an Identification Number can generally refer to various identification systems but does not specify a nine-digit format; a Tax Identification Number often includes SSNs but can also refer to other numbers used for tax purposes such as Employer Identification Numbers (EINs); and a Personal Identification Number is typically a code used for security purposes, like at ATMs, rather than an identification number linked to the individual’s identity. Hence, the Social Security Number stands out as the proper answer to the question.