Aspect ratio
Aspect ratio of picture or display is the ratio between width and height. i.e.:  If a picture or video has resolution of 1920 x 1080, That means the width is 1920 and the height is 1080. Now if we write them as ratio then these two parameters become (width:height) 1920:1080. Dividing both of them by the same denominator 120 it becomes 16 : 9. Which is ultimately the aspect ratio of the picture or video.