SD may refer to any of the following:
1. Short for standard definition, SD is a resolution of video media measuring 720 x 480 pixels and has an aspect ratio of 4:3. Today, SD is the lowest quality, coming in behind the HD, FHD, QHD, and 4K and 8K resolutions (UHD). Consequently, SD is mostly phased out of modern devices.
2. Short for Secure Digital, an SD card is a memory card. See our SD card definition for more information and related terms.
3. Short for single-sided diskette or single-density diskette, SD is a diskette that supports writing on one side of the diskette. See our single-density diskette definition for more information and related terms.