SIM card Subscriber Identity Module
A smart card that identifies the subscriber to the mobile network. First manufactured by Giesecke & Devrient in 1991 for GSM. Holds an IMSI and a secret key that never leaves the card.
Authentication works by the network sending a challenge that the card answers using the stored key — so the key can be used but never read out. The form factors have shrunk in sequence: full-size (credit card sized), mini (25×15 mm, the one you snapped out of a plastic card), micro, nano. The chip itself has stayed the same size the whole time; only the plastic around it has been trimmed away. eSIM moves the same function into a soldered chip provisioned over the air.