Plain English Meaning
How close the VOR's course information is to the truth. Pilots verify this before flying by instruments, because if the indicated course is off by more than a few degrees, the aircraft could be far from where the instruments suggest.
Full Definition
The degree to which a VOR (Very High Frequency Omnidirectional Range) navigation system provides correct course guidance, typically held to a tolerance of plus or minus 4 degrees on the ground during an operational check. VOR accuracy depends on proper ground station calibration, aircraft receiver condition, signal reception quality, and the absence of or along the path.