Elasticsearch
Elasticsearch is the database indexing engine behind Nagios Log Server. It is responsible for handing queries, indexes, and replication. Elasticsearch is ’E’ in the ELK stack.

ELK Stack
The ELK stack is a combination of Elasticsearch, Logstash, and Kibana. These components act as a foundation for a log monitoring solution. Nagios Log Server utilizes the ELK stack.

ESXi Server
VMware ESXi (formerly ESX) server is a enterprise class hypervisor used to run virtual machines. Nagios XI can monitor both ESX hosts and Guest VM’s on ESX and vCenter servers with easy to run VMware monitoring wizard.

Event Logs
Generally refers to log files on Microsoft Windows machines / servers. Many people want to monitor Windows Event logs to watch for security alerts, etc.