What does the acronym SIM stand for?

SIM stands for Subscriber Identity Module. A SIM is a secure component that identifies a mobile subscriber to a cellular network and helps the network authenticate that subscription.

Physical SIM cards store information such as an international mobile subscriber identity, or IMSI, along with security data used during network authentication. This lets a compatible phone connect to its carrier and, traditionally, allows a user to move a subscription between phones by transferring the card.

The term is commonly used for the removable card, although the complete smart-card hardware is technically called a UICC. A SIM may be a full-size card, mini-SIM, micro-SIM, nano-SIM, or an embedded SIM, known as an eSIM. These formats became progressively smaller while preserving the core contact arrangement.

The first SIM cards were manufactured in 1991 by Giesecke+Devrient for Radiolinja's GSM network in Finland. SIM cards are often associated with phone numbers, but the SIM primarily identifies and authenticates the subscription; the phone number itself is handled separately by the carrier's network.