RAW refers to any of several native data formats that contain the full data captured by the sensor. Typically these are proprietary formats, 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, Canon (cr2/crw), Nikon (nef/nrw), Panasonic (raw/rw2), Pentax (ptx/pef), and Sony (arw/srf/sr2).
