Aqua is the graphical user interface, design language and visual theme of Apple's macOS and iOS operating systems.

It was originally based on the theme of water, with droplet-like components and a liberal use of reflection effects and translucency.

Its goal is to "incorporate color, depth, translucence, and complex textures into a visually appealing interface" in macOS applications.

The appearance of Aqua has changed frequently over the years, most recently and drastically with the release of macOS Big Sur in 2020 which Apple calls the "biggest design upgrade since the introduction of Mac OS X.

"[5] For years, Apple had been trying and failing to produce a next-generation Mac OS operating system, including projects code-named Pink, Taligent, and Copland.

[6]: 8–20 Early versions of Mac OS X, called Rhapsody, was a developer release that had an interim user interface, blending MacOS 8's "Platinum" and OpenStep looks.

[7]: 27–28  The Rhapsody approach was ultimately abandoned, and the new operating system was dubbed Mac OS X in 1998.

For instance, toolbars and sidebars are often grey or metal-colored, window backgrounds and popup menus are white and buttons (in older systems also scrollbar handles) are accented with a bright blue.

[citation needed] OS X Yosemite added a dark mode that darkens the predominantly white menubar and Dock.

Aqua windows typically have a metal-like or gray titlebar with three buttons on the left side (for closing, minimizing and zooming or entering fullscreen mode).

[14] Leopard introduced a unified style that extended the metal-like background of the titlebar to the toolbar, making it appear as one whole.

Menus are backed with a slightly translucent solid gray, and when menu items are highlighted they appear blue.

There is also a combination textbox and picker control, which allows the user to type in a date and time or edit it with directional buttons.

In addition, circular, purple online help buttons are available which display help relative to the current task when clicked.

Disclosure triangles, although technically buttons, allow views of controls to be shown and hidden to preserve space.

Radio buttons are similar in appearance and behaviour except that they are circular and contain a dot instead of a check.

The progress bar itself is available in two varieties: indeterminate, which simply shows diagonal blue and white stripes in animation with no measure of progress; or determinate, which shows a blue pulsing bar against a white background proportional to the percentage of a task completed.

The spinning wheel indicator, also found in the Mac OS X startup screen since version 10.2, is simply a series of 12 increasingly darker grey lines arranged circularly, like the side view of a spoked wheel rotating clockwise.

The circular slider is simply a gray dot on a white circle which can be rotated to set values.

In Mac OS X Panther buttons were made to appear sunken into their surroundings, following a general trend of more flattened interface elements in the operating system.

Mac OS X Tiger brought more subtle changes, including the unified titlebar scheme.

The Dock's design, when placed on the left or right side, changed to a black translucent background with a white border and rounded corners.

Most notably, the context menu of Dock items changed from a solid white to a translucent black.

The corner radii of push buttons were reduced, giving an appearance similar to Mac OS 8 and 9.

The applications Calendar, Contacts and Notes respectively lost their leather, book and notepad appearance (all introduced in Lion).

Linen textures in Notification Center and Launchpad were removed as well and replaced with simple gray backgrounds.

[23] In OS X Yosemite, many of the Aqua elements and icons were simplified and flattened, following a similar design change in iOS 7.

Yosemite also added a 'dark theme' you can turn on in the settings which makes the dock and menu bar black.

The white toolbar buttons regained a slightly glossy look, the spinning pinwheel was redesigned and the Vibrancy effect was reduced in certain areas, such as Mission Control.

macOS Big Sur brought major changes to Aqua design that brings it in line with other Apple platforms such as iOS and iPadOS.