Primary account number (PAN)
The 15 to 19 digits on the front of a card, structured enough to give away both the issuer and your typos.
The format is defined in ISO/IEC 7812. The first digit identifies the industry: 4 means Visa, 5 and 2 mean Mastercard. The leading digits form the IIN, which the industry calls the BIN, and point to the issuer. BINs were six digits for decades until they ran out, and eight has been the standard since April 2022. Then comes the account number, and finally a Luhn check digit that catches most typing errors and protects against nothing else.
The PAN is what PCI DSS revolves around. Any system that stores, processes or transmits it lands in audit scope, and wherever it is displayed only the BIN and last four digits may show. Hence the **** 1234 on your receipt, and hence the popularity of tokenization as a shortcut.