Fusion Drive is a type of hybrid HDD/SSD storage technology developed by Apple Inc.
Announced in October 2012 with the release of a new selection of iMac, Fusion Drive claims to offer consumers the benefits of solid state storage technology in terms of performance, with the benefits of traditional HDD storage technology in terms of capacity.
It works by tasking the operating system with identifying the most frequently used files, applications and so on, on an iMac, and moving them into the solid state storage. This means that frequently used software and files will be loaded quicker by the iMac than less frequently access contents.
As its initial offering, Apple offered an option that included either 1 or 3 Terabytes of traditional storage, and up to 768GB of flash storage with a full sized iMac. It also offered a 1TB Fusion drive option for the Mac Mini, that utilized 256GB of solid state storage.