You may be wondering where is the "Meta" key on your keyboard since no key is labeled as such. That key is most probably emulated by the "Windows key" on your keyboard. But why is it sometimes called "Meta"?
Meta key is originated on keyboards of the Lisp computers in 1960 and continued to appear in Sun computers. In the latter computer, it appeared with a diamond-shaped icon. In newer computers, it's Windows key or Command key (sometimes referred to as the cloverleaf key or Cmd key) in Apple computers.
When it comes to keyboard shortcuts, Apple programs use the Cmd key much more than in the Windows platform. Therefore Cmd key is also similar to the Ctrl key in Windows.
Linux world may call this key Meta, as it is not Windows. However, if you have a keyboard with a Windows key, the Meta key may mean the Windows or Alt key on your keyboard. Nowadays, the Windows key is generally used to emulate "Super" key.