what is a tax identification number tin 
a tax identification number tin is a nine digit number used by the internal revenue service irs to identify taxpayers also known as a taxpayer identification number one type of tin includes social security numbers ssns issued by the social security administration ssa investopedia ellen lindnertypes of tax identification numbers tin a child must have an ssn before a parent can claim them as a dependent for income tax purposes so parents voluntarily apply for the number on behalf of their children 6