Glossary

agent: Software service running on the OpenStack infrastructure measuring usage and sending the results to the collector.
API server: HTTP REST API service for ceilometer.
ceilometer: From WikiPedia¹:

A ceilometer is a device that uses a laser or other light source to determine the height of a cloud base.
central agent: Software service running on a central management node within the OpenStack infrastructure measuring usage and sending the results to the collector.
collector: Software service running on the OpenStack infrastructure monitoring notifications from other OpenStack components and meter events from the ceilometer agent and recording the results in the database.
compute agent: Software service running on a compute node within the OpenStack infrastructure measuring usage and sending the results to the collector.
data store: Storage system for recording data collected by ceilometer.
non-repudiable: From WikiPedia²:

Non-repudiation refers to a state of affairs where the purported maker of a statement will not be able to successfully challenge the validity of the statement or contract. The term is often seen in a legal setting wherein the authenticity of a signature is being challenged. In such an instance, the authenticity is being "repudiated".