Application cryptogram (ARQC)
The cryptographic checksum an EMV chip computes for every purchase, which is what makes copied card data worthless.
The chip holds a key unique to the card, derived from the issuer's master key. With it, it computes a MAC over the amount, currency, date, a transaction counter and a random number supplied by the terminal. The eight-byte result is the ARQC (Authorization Request Cryptogram), and it travels with the authorization to the issuer, where a payment HSM recomputes it and answers with its own, the ARPC. If the card approves offline it produces a TC instead; if it declines, an AAC.
Because the cryptogram is tied to one transaction, skimming it gets you nothing. The weak spot is the terminal. In 2014 Bond, Murdoch and Anderson at Cambridge showed that some ATMs generated predictable "random" numbers, allowing cryptograms for future withdrawals to be harvested in advance.