History of Meta Key
The Meta key first appeared on SAIL Keyboard around 1970.
Its function is similar to the Control Key, for entering modified keycodes.
It subsequently appeared on other Lisp Machine Keyboards
It also appeared on Sun Microsystems keyboard.
then, it became Apple's Command key or the Apple key. [see Apple Keyboard History ⌨]
And it became Windows key on Microsoft keyboards and operating system. First appearance is probably the Microsoft Natural Keyboard
Meta key in emacs
The meta key, today is survived by emacs. Emacs is the only app that rely on it extensively, and kept using the jargon meta key. [see Emacs: What is Meta Key]