coun ‧ ter ‧ foil /ˈkaʊntəfɔɪl $ -tər-/ BrE AmE noun [countable]
[ Date: 1700-1800 ; Origin: counter- + foil 'piece of paper, counterfoil' (15-18 centuries) (from Old French ; ⇨ ↑ foil 1 ) ]
the part of something such as a cheque that you keep as a record