Plain English Meaning
When what your aircraft's navigation system shows for a route or procedure does not match what is printed on the official chart or notice. The two sources disagree, and the pilot has to recognize it and resolve it before flying the procedure.
Full Definition
Differences between the navigation information stored in an aircraft's onboard navigation database and the information published on current charts, instrument procedures, or NOTAMs. These differences may involve waypoint names, fix coordinates, altitudes, course values, procedure sequencing, or the presence or absence of specific .