Jailbreaking is a process that allows iPad, iPhone and iPod Touch users to gain root access to the command line of the iOS operating system, thereby removing usage and access limitations imposed by Apple. Once jailbroken,[1] iPhone users are able to download extensions and themes that are unavailable through the App Store (via installers such as Cydia) and perform other tasks that are not possible on store-bought devices, including installing non-Apple operating systems such as Linux.[2] A jailbroken iPad, iPhone or iPod Touch running iOS can still use the App Store and iTunes and other normal functions, such as making telephone calls.

Jailbreaking is different from SIM unlocking,[3] which, once completed, means that the mobile phone will accept any SIM without restriction on, for example, the country or network operator of the owners choice. Jailbreaking, according to Apple, can void Apple's warranty on the device.[4] Under the DMCA of 2010, jailbreaking is perfectly legal, although Apple may claim it to void your warranty. The jailbreaking process can be quickly and easily reversed by restoring the device in iTunes.

Cydia creator Jay Freeman estimates that over 10% of all iPhones are jailbroken.[5]

Cydia (pronounced /ˈsɪdi.ə/) is a software application for iOS that is a graphical front end to APT and the dpkg package management system. It allows a user to browse and download applications for a jailbroken iPhone, iPod Touch or iPad. It was developed by Jay Freeman (also known as “saurik”).[1] Cydia was originally released as an open-source alternative to Installer.app on iOS 1.1.x, but quickly became the most popular package manager upon 2.0's release.

Cydia's repositories allow the installation of software packages, most of which are available to download at no cost.[2] Cydia also includes the Cydia Store, which offers a number of applications for sale. Software packages are downloaded directly to an iOS device, to the same location as Apple's pre-installed applications (the /Applications directory).

The name “Cydia” is an allusion to the Codling Moth (“Cydia pomonella”), which made famous the “worm in the apple.”