Stands for "Secure Digital." It is a type of memory card used for storing data in devices such as digital cameras, PDAs, mobile phones, portable music players, and digital voice recorders. The card is one of the smaller memory card formats, measuring 24mm wide by 32mm long and is just 2.1mm thick. To give the cards some orientation, the top-rght corner of each SD card is slanted. Even though the cards are extremely small, as of late 2004, they can hold up to 8GB of data.
Part of the reason the cards are called "Secure Digital" cards is because the cards have a copyright protection feature built in. The security feature, called "key revocation" means protected data on the card can only be read by specific devices. The cards can have both secured and unsecured areas on them for copyrighted and non-copyrighted data. For more information on SD cards, visit the SD Card Association.