Certificate of purchase
The document issued to the winning bidder at a tax lien sale, proving their claim to the debt. Some states use this exact term (Missouri, for example); others just call it a "tax lien certificate."
See how this plays out in a specific state: browse all 50 states, or check the full glossary for related terms.
General definition — exact terminology and mechanics vary by state and county. Always confirm against the specific county's own auction notice before bidding.