Definition of:in-memory database
in-memory database

Storing the contents of a database in main memory rather than on disk. Since memory (RAM) is considerably faster than a hard disk or even a solid state disk, complex decision support queries can be satisfied much more rapidly, and high-end computers can be configured with terabytes of memory. However, memory can fail due to power outages and component breakdown; therefore, snapshots are taken periodically and saved to permanent disk storage. See SAP HANA and RAM disk.

In Time, It Will Merge
At some point in the future, perhaps 10 or 20 years from now, main memory and disk storage will merge into one "memory/storage" pool as all storage will be addressable like DRAM memory but will be non-volatile. See future memory chips.