Standard Definition (SD) refers to a video resolution that is lower than High Definition (HD). Typically, SD has a resolution of 480p, which means it displays 480 horizontal lines of pixels. This format was commonly used in older television broadcasts and DVDs, providing a basic level of image clarity. While SD is still used in some contexts, it has largely been replaced by higher resolutions like HD, Full HD, and 4K. These newer formats offer improved picture quality, sharper images, and better overall viewing experiences, making them more popular in modern media consumption.