Definition: Secure Digital or SD cards are small (24x32mm) cards that plug into compatible SD slots on consumer electronics devices. They hold hold "flash" memory that is retained even when the device is turned off. SD cards may hold additional memory ranging from eight megabytes to 16 gigabytes. SD cards for GPS devices often come pre-loaded with supplemental maps or charts to greatly enhance map detail and supplemental travel information. SD cards may also be used for media storage. A "micro" SD card performs the same functions, but is much smaller (15x11mm) than an SD card, and is designed for smaller (handheld and smaller) GPS devices.