The deltoid is the three headed muscle of the shoulder. The lateral, or side head, is situated in the middle of the three, to the side of the shoulder. The full title of the deltoid is deltoideus, and is named after the triangular Greek letter; Delta.
Bodybuilders and those seeking to improve their aesthetics will often place importance on the development of the side deltoid, with it being responsible with the general width of the shoulders which is important for the desirable v-taper.
The side shoulder head is primarily targeted during abduction of the shoulder, or in other words, when the upper arms are moved up away from the side of the body, like during lateral dumbbell raises.