A number that is assigned to a tax paying businesses for identification and record keeping purposes. The TIN number allows the IRS to keep track of all taxpaying entities and manage their accounts. Corporations, estates and trusts must file their taxes under this number each year.

For firms, the TIN number is their Social Security number equivalence. For corporations, this number is their EIN (Employer Identification Number). Estates, trusts and partnerships use EINs as well.