Aspect ratio refers to the proportional relationship between the width and height of an image or screen. It is typically written as two numbers separated by a colon, such as 16:9 or 4:3. The first number represents the width of the image or screen, while the second number represents the height. Aspect ratio is an important consideration in graphic design, as it affects the overall look and composition of the image. Different aspect ratios can be used to achieve different visual effects, and can also impact how the image is displayed on various screens or devices.