A type of removable flash memory card designed specifically for mobile phones.
Like any flash memory card, it can be used for long-term storage of various types of files, including photos, videos, music, or software.
microSD is one of the smallest memory card formats available; a microSD card is about the size of a fingernail. It was designed to be smaller than competing formats, to allow phones using the format to be smaller overall. It is the most common type of removable memory card used in phones.
There are adapter cards that allow a microSD card to be used in a full-size SD card slot. This is a convenient way to use a microSD card with a card reader on a PC, to transfer files between a PC and a phone.
microSD cards with a memory capacity larger than 2 GB are microSDHC, which uses the exact same technology as SDHC, just in the smaller microSD size. The "HC" stands for "high capacity". These cards can only be used in microSDHC-compatible devices. Some older phones do not support microSDHC, and therefore do not support cards larger than 2 GB.
A newer standard called UFS has also been introduced, which is similar but faster:
The microSD card format was first brought to market in 2004 as "TransFlash". In 2005, the format morphed into "microSD". It was mostly just a name change, as all microSD cards should be compatible with TransFlash slots and vice-versa. The format has also gone by the names "T-Flash" and "TriFlash-R".