iOS (formerly known as iPhone OS) is Apple's mobile operating system. Originally developed for the iPhone, it has since been extended to support other Apple, Inc. devices such as the iPod touch, iPad and Apple TV. Apple, Inc. does not license iOS for installation on third-party hardware. As of October 4, 2011, Apple, Inc.'s App Store contains more than 500,000 iOS applications, which have collectively been downloaded more than 18 billion times. In the last quarter of 2010, it had a 26% share of the smartphone operating system market in terms of units sold, behind Google's Android and Nokia's Symbian. As of May 2010, it accounted for 59% of mobile web consumption—including both the iPod touch and the iPad—in North America.

The user interface of iOS is based on the concept of direct manipulation, using multi-touch gestures. Interface control elements consist of sliders, switches, and buttons. The response to user input is immediate and provides a fluid interface. Interaction with the OS includes gestures such as swipe, tap, pinch, and reverse pinch, all of which have specific definitions within the context of the iOS operating system and its multitouch interface. Internal accelerometers are used by some applications to respond to shaking the device (one common result is the undo command) or rotating it in three dimensions (one common result is switching from portrait to landscape mode).

iOS is derived from Mac OS X, with which it shares the Darwin foundation, and is therefore a Unix-like operating system by nature.

In iOS, there are four abstraction layers: the Core OS layer, the Core Services layer, the Media layer, and the Cocoa Touch layer. The current version of the operating system (iOS 5.0.1) uses roughly 774.4 megabytes of the device's storage, varying for each model.

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