Secure Digital

Secure Digital (SD) is a non-volatile memory card format developed by the SD Card Association (SDA) for use in portable devices. The standard was introduced in August 1999 as an improvement over MultiMediaCards (MMC), and has become the de facto industry standard.

Secure Digital includes four card families available in three different form factors. The four families are the original Standard-Capacity (SDSC), the High-Capacity (SDHC), the eXtended-Capacity (SDXC), and the SDIO, which combines input/output functions with data storage. The three form factors are the original size, the mini size, and the micro size. Electrically passive adapters allow a smaller card to fit and function in a device built for a larger card.

There are many combinations of form factors and device families, although as of 2013, the prevailing formats are full- or micro-size SDHC and full or micro SDXC.

The SDA uses several trademarked logos to enforce compliance with its specifications and assure users of compatibility.