The Meta key is a modifier key on certain keyboards. It first appeared on the Stanford keyboard and successors such as the Knight keyboard, space-cadet keyboard (where it is labeled “META”), MIT Lisp machine, Symbolics keyboards (where it is labeled “META” or “Meta”), and on Sun Microsystems keyboards (where it is marked as a solid diamond “◆”).
Generally the Meta key worked similar to Macintosh's Command key, in that when held down it modified letters and symbols into immediate commands (shortcuts). On these keyboards you could get many commands per letter by combinations of Meta with other modifier keys such as Hyper and Super.
One notable difference from the other keys, dating to the Knight keyboard, is that the Meta key was outside the Control key, instead of inside the Control key (as the Alt key is usually placed). This results in modifier keys for Emacs keybindings that were designed for Lisp machine keyboards being swapped on standard keyboards.
On keyboards that lack a physical Meta key, its functionality may be invoked by other keys such as the Windows key. Software often provided another workaround, such as using the Alt key (which does not exist on the Knight keyboard), or using the Esc key as a prefix (for example in Emacs). Because of these workarounds the need for Meta, despite being the most-used additional shift key, was less than for other shift keys. Thus it is more common today to use the Windows key to emulate the Super key.