A counterfoil is a document that is used to verify the authenticity of a transaction. It is usually a duplicate of the original document, such as a RECEIPT, DOCKET, or STUB. The term 'counterfoil' is derived from the fact that it is a foil to the original document, providing an additional layer of security. In a crossword puzzle, the solutions for the term 'counterfoil' could include 'RECEIPT' (7 letters), 'DOCKET' (6 letters), 'STUB' (4 letters), and 'stubs' (5 letters).