Taxpayer Identification Number, the umbrella term for the numbers the IRS uses to identify taxpayers: an EIN for a business, an SSN for an individual, and a few rarer types. A vendor's W-9 exists to get their TIN on file correctly.
In Bizdivers: TINs extracted from W-9s are stored masked, and a missing TIN on a threshold-crossing vendor is flagged as a 1099 exception. See Collect and manage vendor W-9s.