SD (Secure Digital) - definition
Secure Digital or SD is a popular type of memory card. Memory cards are based on Flash memory and are used as solid state storage, their main purpose being an inexpensive way to store or transfer data between computers and other digital products.
The dimensions of an SD card are 24 x 32 x 2.1 mm, which makes it smaller than CompactFlash but larger than miniSD, microSD (TransFlash) and RS-MMC.
SD cards are about the same size as MMC but a bit thicker. They provide a bit more speed than MMC at the expense of higher power consumtion. SD cards also provide a hardware lock that can protect the stored data from being erased.