Nav
Nav turns up in 2 fields: hydrography, maritime. Here is what it means to the people in each one.
Nav in hydrography
Shorthand for the vessel's positioning data stream, or for the positioning system as a whole.
Survey-room shorthand for everything to do with position: the navigation file, the GNSS and inertial feed and the helm display. Crews talk about nav jumps when position leaps suddenly, nav spikes in the processed track, and swapping post-processed nav in for the real-time version. Bad nav is diagnosed by plotting the trackline and looking for zigzags, steps or gaps.
"Nav jumped 6 metres at the end of line 12, check whether that's multipath off the ship's hull."
Nav in maritime
A common shorthand for navigation, often used to describe equipment, lights, or duties.
Nav is the everyday clipped form of navigation. Crews use it constantly as an attributive noun to describe anything related to charting a course, from the nav station on the bridge to the nav lights required at night. It saves syllables on the radio and in quick commands.
"Make sure the nav lights are on before we clear the breakwater."