RADIUS – a Remote Authentication Dial-In User Service is a protocol for administering and securing remote access to a network. It needs an authentication server, client protocols and an accounting server, all of which can be mounted on a single machine. The RADIUS authentication server validates the user credentials (user name and password) before allowing access. It can provide different levels of user privileges. It does not provide encryption.
RAID – a Redundant Array of Inexpensive Disks uses an array of disks instead of one large expensive one. A single disk in the array can be removed and the data remains safe. RAID level 5 provides the highest level of safety.
RDMBS – Relational Database Management System.
Reboot – this is what you have ...