The Apple Watch operating system, introduced in April 2015. In the same year, watchOS 2 added a variety of features, including an enhanced Siri, more watch faces, support for third-party apps that run without pairing to the iPhone and an alarm clock/nightstand mode.
In 2016, a more efficient watchOS 3 debuted with Apple's second-generation smartwatches. New watch faces and several features were added, including more Siri integration, improved social apps and a personal alarm that dials an emergency number.
In 2017, watchOS 4 added more Siri-based watch faces for daily news and reminders, a changing kaleidoscope face and several animated Toy Story faces. Activity apps are enhanced, supporting multiple workouts, more personalized notifications, multiple workouts in a single session and integration with major gym equipment. Music app more integrated with workouts.
In 2018, watchOS 5 added a walkie talkie to send messages to other Apple Watch users via Wi-Fi or cellular. It added automatic workout detection that starts tracking if the wearer forgets to start the app. watchOS 5 requires a minimum of an iPhone 5S, iOS 12 and second generation Apple Watch. See Apple Watch