The Super key refers to several different keys throughout keyboard

Keyboard (computing)

In computing, a keyboard is a typewriter-style keyboard, which uses an arrangement of buttons or keys, to act as mechanical levers or electronic switches...

 history. Originally the super key was a modifier key

Modifier key

In computing, a modifier key is a special key on a computer keyboard that modifies the normal action of another key when the two are pressed in combination....

 on the Space-cadet keyboard. Recently with Windows and Macintosh

Macintosh

The Macintosh , or Mac, is a series of several lines of personal computers designed, developed, and marketed by Apple Inc. The first Macintosh was introduced by Apple's then-chairman Steve Jobs on January 24, 1984; it was the first commercially successful personal computer to feature a mouse and a...

 computers doing away with large numbers of modifier keys the Super key has become an alternative name for other modifier keys when using a Linux

Linux

Linux is a Unix-like computer operating system assembled under the model of free and open source software development and distribution. The defining component of any Linux system is the Linux kernel, an operating system kernel first released October 5, 1991 by Linus Torvalds...

 or BSD based operating systems.

Knight Keyboard to the Space-cadet Keyboard

Tom Knight created both the Knight keyboard and the Space-cadet keyboard. Among the improvements from the Knight keyboard to the Space-cadet keyboard was the presence of two more modifiers for the bucky bits, hyper and super; The Super Key modified the third bucky bit (representing a 4). From the usage of the Space-cadet keyboard on LISP machines the super key was inherited by Emacs

Emacs

Emacs is a class of text editors, usually characterized by their extensibility. GNU Emacs has over 1,000 commands. It also allows the user to combine these commands into macros to automate work.Development began in the mid-1970s and continues actively...

 as one of several supported modifier keys, most modern systems have to emulate the super key using another.

Linux and BSD

In most non-Windows operating systems the super key is mapped to the Windows key

Windows key

The Windows logo key—also known as the Windows key, the home key, the meta key, the start key, MOD4, or the flag key —is a keyboard key which was originally introduced on the Microsoft Natural keyboard before the release of Windows 95. This key became a standard key on PC keyboards...

 and is the preferred term for that key.

In GNOME

GNOME

GNOME is a desktop environment and graphical user interface that runs on top of a computer operating system. It is composed entirely of free and open source software...

 Super_L and Super_R are mapped to the left and right windows keys respectively and are treated as modifiers. Under GNOME 3 letting go of the Super key defaults to showing the activities window, which appears to ruin the ability to use some other super key combinations.

In Openbox

Openbox

Openbox is a free window manager for the X Window System, licensed under the GNU General Public License. Originally derived from Blackbox 0.65.0 , Openbox has now been totally rewritten in the C programming language and since version 3.0 is not based upon any code from Blackbox.Openbox is designed...

 and KDE

KDE

KDE is an international free software community producing an integrated set of cross-platform applications designed to run on Linux, FreeBSD, Microsoft Windows, Solaris and Mac OS X systems...

 the super key is an available modifier key

Modifier key

In computing, a modifier key is a special key on a computer keyboard that modifies the normal action of another key when the two are pressed in combination....

 but is not used in any default shortcuts. Under Unity the key is used to control launcher and manage windows.