Secure Digital (also known as SD) is a flash memory memory card format. SD cards are based on Toshiba's older Multimedia Card (MMC) format, but add little-used DRM encryption features and allow for faster file transfers, as well as being physically slightly thicker. Devices with SD slots can use the thinner MMC cards, but SD cards won't fit into the thinner MMC slots. Standard SD cards measure 32 mm by 24 mm by 2.1 mm.
SD cards are currently available in sizes up to and including 2 GB, and are used in almost every context in which memory cards are used, and in nearly every application, they are the most popular format. SD support is standard in PDAs, with Dell, palmOne, HP, Toshiba, Sharp, and others including SD slots in all of their PDAs. Digital cameras (including Kodak's cameras) tend to support SD cards, as well, although Olympus and Fuji (with xD cards) and Sony (with Memory Sticks) favor their own proprietary formats, and many professional cameras support CompactFlash in addition to or instead of SD, to allow pro photographers to use multi-gigabyte microdrives. (Sony has started to include secondary SD slots on some newer cameras.) SD cards can also be found in flash-memory-based mp3 players, GPS receivers, the occasional cell phone, and occasional portable game systems.