Definition: Secure Digital (SD) is a type of flash memory card used in digital cameras to store data about images. SD is probably the most popular type of memory card. SD and MMC flash memory cards are similarly sized and use similar technology as SD's design was based on MMC's design, but not all digital cameras can use the two types of cards interchangeably. MiniSD and MicroSD cards are available, too, but they typically work with cell phones, rather than digital cameras. Adapters are available to make a MiniSD or MicroSD card work in a digital camera that uses SD memory cards.
Matsushita, SanDisk, and Toshiba combined to develop Secure Digital cards in 1999. SD cards are slightly rectangular, with one corner cut at a 45-degree angle. SD cards measure 24 mm by 32 mm by 2.1 mm (0.94 x 1.26 x 0.08 inches).
Because of a change in the SD technology, SDHC (SD high capacity) cards now allow storage capacities at 4GB (gigabytes) and beyond. SD cards are limited to 2GB. Some older digital cameras that use SD cards cannot read SDHC cards.
