The term 'stereo button' refers to two different types of buttons found on stereo systems. The first is the PHONO button, which is a five-letter word that stands for phonograph. This button is used to switch between the stereo's built-in speakers and an external device, such as a record player. The second is the AUX button, which is a three-letter word that stands for auxiliary. This button is used to switch between the stereo's built-in speakers and an external device, such as a smartphone or MP3 player. Both of these buttons are used to control the sound output of the stereo system.