Short for Alternate key, the Alt key is like a second Control key. Not all computer keyboards have an Alt key, but it is standard on all PCs. You use it in the same fashion as the Control key — holding it down while you press another key. For example, an instruction to use the Alt+P combination means that you should hold the Alt key down while pressing and then releasing the P key. The meaning of any Alt key combination depends on which application is running.
On Macintoshes, the equivalent key is called the Option key.