Taxpayer Identification Number (TIN) Definiton
A nine-digit account number representing a person or business for the IRS. This can be a Social Security number (SSN), Employer Identification Number (EIN), or an Individual Taxpayer Identification Number (ITIN).
For individuals, the Taxpayer Identification Number (TIN) is their Social Security Number (SSN). Taxes for corporations, trusts, and estates must file taxes under their TIN each year. In addition to being a necessary identifier when filing taxes, this number is also required when opening bank accounts.