watchOS is the operating system designed specifically for Apple Watch, Apple's first foray into wearable computing devices. This tag should be used for questions relating to the software used on Watch.
watchOS 3, launched in September 2016, overhauled the OS and was pointed to by many observers as the software that Apple Watch should have launched with. Even on devices that were by then, over a year old, performance was greatly improved.
This tag is for any and all questions related to the watchOS operating system software. For Watch hardware, use apple-watch.