microSD is a format for removable flash memory cards. SD is an acronym for Secure Digital. It is commonly used in cellular phones, but also in handheld GPS devices, portable media players, digital audio players, expandable USB flash memory drives, and for Nintendo DS flashcards, along with digital cameras.
It is the smallest memory card available commercially. It is a quarter the size of an SD card.
There are adapters which allow a microSD card to be used in devices intended for SD, miniSD, Memory Stick Duo and even USB cards. However, they are not universally compatible. TransFlash and microSD cards are the same (each can be used in devices made for the other), except that microSD adds support for SDIO mode, enabling non-memory cards like Bluetooth, GPS, and Near Field Communication devices.
See also What is a miniSD card?
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