(Subscriber Identity Module lock) A function in GSM cellphones that restricts the phone's use to a particular cellular carrier. A phone that is SIM locked may be unlocked by entering a code, and that code may or may not be available from the carrier at the end of the subscriber's contract.

Apple's iPhone brought the SIM lock into public view because of its exclusivity with AT&T in the U.S. However, iPhones have been readily "unlocked" by hackers both in and outside the U.S., not only to use a different carrier, but to customize their appearance and download applications even before Apple debuted its App Store. In addition, Apple sells the iPhone without the SIM lock in Australia, Taiwan, Hong Kong and other regions, where locked phones are either prohibited or are a recipe for failure. See jailbreak and iPhone.