SD may refer to any of the following:
1. Short for standard definition, SD is the quality of a video. Today, SD is the lowest quality of video when compared against HD (high definition).
2. Short for Secure Digital, an SD card is a memory card. See our SD card for a full explanation on this term.
3. Short for single-sided diskette or single-density diskette, SD is a diskette that supports writing only on one side of the diskette. See our single-density diskette definition for a full explanation and related terms.