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The graphical control element accordion is a vertically stacked list of items, such as labels or thumbnails. Each item can be "expanded" or "stretched" to reveal the content associated with that item. There can be zero expanded items, exactly one, or more than one item expanded at a time, depending on the configuration.
Contents
The term stems from the musical accordion in which sections of the bellows can be expanded by pulling outward.
A common example of an accordion is the Show/Hide operation of a box region, but extended to have multiple sections in a list.
An accordion is similar in purpose to a tabbed interface, a list of items where exactly one item is expanded into a panel (i.e. list items are shortcuts to access separate panels).
Developer definition
Several buttons or labels are stacked upon one another. At most one of them can be "active". When a button is active the space below the button is used to display a paned window. The pane is usually constrained by the width of labels. When opened it shifts labels under the clicked label down according to the height of that window. Only one button or pane combination can be active at any one time; when a button is selected any other active panes cease to be active and are hidden. The active pane may have scrollbars.
Purpose
User definition
Several windows are stacked on each other. All of them are "shaded", so only their captions are visible. If one of them is clicked, to make it active, it is "unshaded" or "maximized". Other windows in accordion are displaced around top or bottom edge.
Examples
A common example using a GUI accordion is the Show/Hide operation of a box region, but extended to have multiple sections in a list.
SlideVerse is an accordion interface providing access to web contents.
The list view of Google Reader also features this.
Apple has some roll-over accordions. For example (as of December 2008), the left column of the page includes three categories that expand on roll-over: "All Downloads", "Top Apple Downloads", and "Top Downloads".