Apple's operating system crafted specifically for mobile devices.
The underlying structure of Mac OS X (now macOS) was adopted by iOS, however, the implementation provides a fairly radically different user experience than Mac OS X. Over time, features and design idioms are cross-pollinating with many features from iOS finding their way to macOS beginning in Mac OS X 10.7 "Lion" with features like Launchpad, culminating in newer versions like OS X 10.10 "Yosemite" with APIs that allow handing off editing documents in supported applications between Mac and iOS devices.
Once it became clear that iPhone OS was too unwieldy, the term ios was born and has been used since iOS 4. iPhone, iPod touch, iPad, Apple TV (from version 2 forward) and the latest entrant Apple Watch, all run on the same core OS, though functionality is much more limited on Apple TV. The sharing of the core architecture allows functions such as airplay to work with little to no configuration required by the user.
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.