Noun
From left to right: A keyboard's control key, Windows key, alt key.
alt key (plural alt keys)
(computing) A key on a personal computer or terminal keyboard used in combination to alter the function of other keys
One must hold down the Alt key while pressing this “dual-function” key to invoke SysRq. Source: Internet
This is the US-International layout, which uses the right Alt key as an AltGr key which supports many additional characters directly as an additional shift key. Source: Internet
The US keyboard layout has a second Alt key instead of the AltGr key and does not use any dead keys ; this makes it inefficient for all but a handful of languages. Source: Internet
Alt key + left mouse button immediately drops the full stack. Source: Internet