RAW refers to any of several native data formats that contain as close to the full amount of data captured by the sensor as the manufacturer allows you to reach. Typically these are proprietary formats based on TIFF, CIFF, or JPEG packing, and each company uses its own.
While RAW is not an acronym, it is usually written in all-caps in the manner of other image file formats, like TIFF, GIF, JPG/JPEG, and PNG. This helps distinguish the technical term from the English-language word.
There are many types of RAW formats, including, but not limited to
See also Wikipedia on Raw image format, which includes a more comprehensive list.