The operating system in Apple's smartwatch. Introduced in April 2015, the second generation OS came out in the same year.
As of watchOS 7, apps can display relevant information on the watch face; for example, outside temperature, map location or foreign language translator. Complications, which is a strange name for this feature, are like widgets on a desktop computer. 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.