RAM Disk
From Wikipedia:
A RAM drive (also called a RAM disk) is a block of RAM (primary storage or volatile memory) that a computer’s software is treating as if the memory were a disk drive (secondary storage). With Nagios installations massive performance benefits have been achieved by using a RAM disk to reduce disk I/O. Utilizing a RAM Disk in Nagios XI

Redundant / Redundancy
Generally the same as Faiiover. See Faiiover.

Remote Network
Generally refers to remote LANs that contain devices / servers / workstations the users want to monitor. Remote networks might be connected to the central LAN or NOC with a VPN.

Root User
The root user on a Linux/UNIX server is considered to be the "superuser" or administrator of the system. The root user is able to run any and all commands on a system - including starting, stopping, and restarting services, reconfiguring system parameters, managing file and directory permissions, and managing user permissions.

Router
A network device that is used to "route" data packets between computer networks. Example: A router is used to connect an office network to the greater internet.

RRD File (round-robin database file)
Used to store and compress time series data such as Nagios plugin returned performance data. Nagios XI stores all of the performance data that create graphs in RRD Files.