odule lock) A function in GSM cellphones that restricts the phone's use to a particular cellular carrier identified by a SIM card. SIM locks are used to ensure subscribers finish their service contracts. If the contract period is over, the phone may be unlocked by entering a code from the carrier.
Apple's iPhone brought the SIM lock into public view because of its original exclusivity in the U.S. with AT&T. However, iPhones were 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 2008. In addition, Apple allowed the GSM iPhone to be sold without the SIM lock in Australia, Taiwan, Hong Kong and other regions where locked phones were either prohibited or a recipe for marketing failure. See SIM card
and iPhone jailbreaking