SIM
The subscriber identity module holds a subscription's identity and authentication keys, and its form factor is packaging.
Definitions only — a term explained here is not a service we sell.
A SIM holds the subscription identity and the authentication keys a device uses to join a network. Removable cards, soldered MFF2 chips and eSIM profiles carry that identity in different form factors.
The form factor is packaging. The identity, keys and algorithms sit in the UICC, so a soldered chip and a plastic card can present the same subscription.
What the card carries
The ICCID is the card's own serial number. The IMSI is the subscriber identity the card presents. The phone number belongs to the subscription in the network, not to the card, which is why a SIM swap can move the number to a new card; the number follows the subscription, while the ICCID belongs to each card.
An eUICC adds remote profile management, so the subscription can change without a physical swap. An eSIM is a profile on that chip, not a different kind of subscription. The What is eSIM guide covers the profile layer.