Standard definition, or SD, refers to the resolution of an image or video that is lower than high definition (HD). Standard definition typically has a resolution of 720 x 480 pixels in the United States, while in Europe it is 720 x 576 pixels. The aspect ratio for SD is 4:3, which means that the width of the image is 4 units and the height is 3 units. This resolution is lower than that of high definition, which typically has a resolution of 1920 x 1080 pixels.
https://en.wikipedia.org/wiki/Standard-definition_television