Active Check
A check that is initiated and performed by Nagios Core or Nagios XI - usually on a pre-determined schedule. Plugins are used to perform active checks.

Agent
Software that is installed on servers/workstations so they can be monitored. Agents are available for Windows and Linux machines/servers. In contrast, agentless monitoring refers to the ability to monitor a machine without having to install an agent on it - usually using SNMP or WMI.

AIX
A UNIX operating system. There are plugins for monitoring AIX with Nagios Core and Nagios XI. See Nagios Exchange.

AngularJS
AngularJS is a client-side Javascript library and development framework use in web applications. It is used by Kibana and is part of Nagios Log Server. The panels in Nagios Log Server dashboard are developed using AngularJS.

Apache
An Open Source web server that can run on Linux and Windows. Nagios Core and Nagios XI are both capable of monitoring Apache, and Apache is used on both Core and XI server to provide the web interfaces to each.

Auto-Discovery Wizard
Nagios XI has a builtin Auto-Discovery wizard. When running an Auto-Discovery job the user will enter an network of addresses they would like to scan and a job is created which utilizes fping and nmap to scan the network defined for alive machines and then performs an nmap scan on the host to determine which ports it is listening on. The results of the Auto-Discovery scan additionally report the OS of the machine discovered as well as all of the ports the host is listening on. A user can then optionally pass these results into a wizard that will monitor to make sure all of these ports continue to be responsive.

Availability
A measure of what percentage of time (or how much time) a service, application, or device was working properly. Nagios XI and Core both have availability reports. See also SLA