TIN - Taxpayer Identification Number is a unique combination of letters or numbers, assigned by a country to an individual or an Entity and used to identify the individual or Entity for purposes of administering the tax laws of such country.
In the context of US, TIN is a nine-digit identification number. It can either be a Social Security Number (SSN) issued to individuals or an Employer Identification Number (EIN) issued to individuals / entities.
In the case of countries listed by OECD (other than US), TIN must be the tax identification number as specified in OECD site or its functional equivalent. Examples of functional equivalents include social security/insurance number, citizen/personal identification/services code, national identification number, a resident / population registration number, Alien card number, etc.
For other countries (except OECD listed countries), any Government ID can be reported as TIN.