SD Definition Stands for "Secure Digital." SD is a type of memory card used to store data in portable electronic devices. Examples include digital cameras, video recorders, smartphones, and portable music players. SD cards are considered removable storage (instead of internal or external storage), since they can be inserted and removed from a compatible device. The first SD cards were introduced in 1999. They used almost the same form factor as the existing MultiMediaCard (MMC) format, b...