a technology developed by Apple to allow a standard spinning hard drive (HDD) to be combined with a solid state drive (SSD) into one logical volume. OS X then manages the files and moves the most accessed files onto the SSD for better performance.
The Fusion Drive is currently only officially supported on the 2012 (and later) iMacs and Mac minis. However, many people have been successful in creating their own Fusion Drive using the diskutil command line tool. The Fusion Drive technology is only supported in OS X Mountain Lion 10.8.2 or later. Older versions of OS X cannot read a Fusion Drive.