SD, or Standard Definition, refers to a video or image resolution that is lower than High Definition (HD) resolution. SD video is typically displayed at a resolution of 640x480 pixels, and is characterized by its lower level of detail and clarity compared to HD video. SD video is often used in applications where the need for high-resolution video is not critical, such as low-quality streaming video, or video that will be displayed on older or low-resolution displays. While HD and UHD (Ultra HD) video have become increasingly common, SD video is still widely used in many applications, and is supported by a wide range of devices and software.