Definition
NAVIGATION REFERENCE SYSTEM (NRS)
The NRS is a system of waypoints for use within the United States for flight planning and navigation without reference to ground based navigational aids. These waypoints are located in a grid pattern along defined latitude and longitude lines and are available for use at or above FL180 by aircraft capable of point‐to‐point navigation.
Source: FAA Pilot/Controller Glossary (verbatim from the Federal Aviation Regulations).