Apple's operating system created for mobile devices.
The term iOS came about after the iPhone was originally released once it became clear that iPhone OS and iPod Touch OS were too unwieldy. The iPad and second version of Apple TV (without an internal hard drive) all run on the same core OS.
Each device gets a customized build even when the major build numbers (e.g. iOS 5.0.1) are the same and the software is released and documented as one build.