The Mac OS X Public Beta first made Apple’s Aqua interface style publicly available.
Code-named Kodiak, the Public Beta was released on September 13, 2000, as a preview of Apple’s next-generation operating system. It gave users and developers an early look at the interface that would ship with Mac OS X 10.0 the following year. Aqua was designed around vivid color, depth, translucency, and water-inspired visual effects.
The interface introduced distinctive elements that became closely associated with OS X, including the Dock, glossy buttons, translucent menus, and animated window behavior. Steve Jobs presented Aqua as both visually appealing and approachable, while Apple used it to signal a dramatic break from the older Platinum appearance of Mac OS 8 and Mac OS 9.
The Public Beta itself was not the final operating system: it expired on May 14, 2001, roughly two months after Mac OS X 10.0 launched. Platinum is the main historical alternative people confuse with Aqua, but Platinum belonged to the classic Mac OS era and some transitional Rhapsody releases.