Plain English Meaning
When you're flying between airports using GPS, the needle that shows whether you're left or right of your route is set to a wider scale: if the needle goes all the way to one side, you're 5 miles off course.
Full Definition
The default sensitivity setting of the Course Deviation Indicator (CDI) when a GPS-equipped aircraft is in the en route phase of flight, in which full-scale deflection of the CDI needle represents 5 nautical miles of lateral displacement from the desired course on either side of centerline.
Context Anchor
Seen in GPS discussions, especially when comparing , , and CDI scaling.