The operating system in Apple's smartwatch. Introduced in April 2015, the second generation OS came out in the same year.
The strangest naming for a feature, a complication on an Apple watch is like a widget on a desktop computer. It allows apps to display relevant information on the watch's face. Following are the features of subsequent OS versions. See Apple Watch
watchOS 7 (2020)
A major release, some features require at least Apple Watch series 3 while others require 5 and above. An iPhone is no longer mandatory to use all Apple watch features, and multiple complications can be displayed. Native sleep tracking and a new fitness app were added. A 20-second hand washing countdown is included, and the noise alert was expanded for headphones.
watchOS 6 (2019)
The app store was added to the watch along with more watch faces, message Animojis and improved health and fitness capabilities. A sound monitor alerts users that the current ambient noise can be harmful. An iPhone 6s or SE with iOS 13 is the minimum requirement.
watchOS 5 (2018)
A walkie talkie was added to send messages to other Apple Watch users via Wi-Fi or cellular. 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.
watchOS 4 (2017)
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.
watchOS 3 (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.
watchOS 2 (2015)
A variety of features were added, 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.