Hybrid storage is used when you store part of your data on solid state drive and part of it on a classical rotating harddisk. Popular use of the term seem to have originated from the Seagate hybrid drives which used on-board flash to store the most read sectors.
It can also be used when combining these features in other ways. E.g:
- ZFS and ARC.
- Hardware RAID cards and flash drives for the same goal. (E.g. maxIQ, cacheVault)
- Software (driver) solutions involving a regular HDD and SSD to 'accelerate' them. (E.g. IRST).