macOS Sierra is an operating system developed by Apple Inc. for its line of Mac computers. Released in September 2016, it introduced several new features, including the ability to unlock a Mac using an Apple Watch and the integration of Siri, Apple's virtual assistant, into the desktop environment. Another significant addition was the Universal Clipboard, which allows users to copy content on one Apple device and paste it on another. macOS Sierra also improved the iCloud Drive experience, making it easier to access files across devices, and introduced the Optimized Storage feature to help manage disk space efficiently.