macOS Sierra is the next generation operating system for Apple's MacBook, MacBook Air, MacBook Pro, Mac mini, iMac, and Mac Pro. It was previewed for the first time at WWDC 2016 on June 13. Previously code named "Fuji", it's been in development since last year. For 15 years, Apple used the OS X nomenclature, but macOS Sierra marks the unification of all Apple software platforms.
In keeping with the theme of using California-based geographical locations, Apple named the latest Mac operating system after the 400-mile long mountain range that runs through Central and Easter California, Sierra. Incidentally, it runs through Yosemite Valley, which creates the connection to Apple's previous Mac operating systems, Yosemite and El Capitan.
Apple has added significant features to Sierra, including support for Siri, Apple Pay on the web, Auto Unlock using Apple Watch, and more, making the Mac more connected to iOS than ever before.