What Does Alternate Key (Alt Key) Mean?
An alternate key (Alt key) is a key that is present on most computer keyboards and is considered a modifier key that can used similarly to the shift or control keys. In other words, the alternate key provides alternate input and operations when pressed in combination with other keys.
Techopedia Explains Alternate Key (Alt Key)
Although the key is standard for all personal computers, not all computer keyboards have the alternate key. The equivalent of the Alt key on Macintoshes is the option key. On most keyboards the Alt key is located on either side of the space bar. However, on some keyboards there is only one Alt key. Any instruction to use the combination of the Alt key and any other keys means that one should hold the Alt key down while pressing and releasing the other keys.
The Alt key can be used for actions like printing special characters, application-specific actions or keyboard-specific functionalities. The function of the Alt key can vary from application to application. Some word processing programs do not recognize certain key combination provided by the Alt key.