(Secure Digital Memory Card) A flash memory card that provides storage for digital cameras, cellphones and PDAs. By 2007, it became the most popular format for point-and-shoot, digital cameras. Introduced in 1999 by Panasonic, Toshiba and SanDisk, cards up to 64GB are available (see SDHC below).

SD Cards use the same 32x24mm form factor as the earlier MultiMediaCard (MMC), but are slightly thicker (2.1mm vs. 1.4mm), and SD Card readers accept both formats.

Fast Storage
Although SD Cards support encryption and content protection (the "Secure" in SD), they have been mostly used for regular storage due to their small size and fast transfer rate (10 and 20 MB/sec). SD uses NAND flash technology (see flash memory).

miniSD and microSD Cards
Miniature variations of the SD Card were introduced with dramatically smaller footprints: the miniSD in 2003 and the microSD in 2005, the latter previously called "TransFlash." Electrically, as well as software compatible, miniSD and microSD cards fit into a regular SD slot via an adapter (see below).

SD High Capacity (SDHC)
SD High Capacity (SDHC) cards provide storage from 4GB to 64GB. Based on the SD Card Association 2.00 specification, SDHC cards do not work in SD devices, because SD uses the FAT16 file system, and SDHC uses FAT32. However, SDHC devices support both SDHC and SD Cards. See Eye-Fi, gruvi card, SDIO card and MultiMediaCard.