watchOS is the operating system of the Apple Watch, developed by Apple Inc.. It is based on the iOS operating system and has many similar features. It was released on April 24, 2015, along with the Apple Watch, the only device that runs watchOS. Its API is called WatchKit. The second version, watchOS 2, included support for native third-party apps and other improvements, and was released on September 21, 2015. The third version, watchOS 3, was released on September 13, 2016, emphasizing better performance and including new watch faces and stock apps. The fourth version, watchOS 4, was released on September 19, 2017. The fifth version, watchOS 5, was released on September 17, 2018, adding more third-party support and new workouts, along with the “Walkie-Talkie” feature. The sixth version, watchOS 6, was released on September 19, 2019.