Plain English Meaning
The needle has moved as far as it can go to one side of center. Once it reaches the edge of the scale, the instrument cannot show how much further off course the aircraft really is.
Full Definition
The maximum displacement of a course or glideslope needle from its centered position on a navigation indicator. On a localizer, full-scale deflection represents an angular displacement of approximately 2.5° from the centerline; on a glideslope, approximately 0.7° from the on-path angle. When the needle is pinned against the edge of the scale, the aircraft is at or beyond this maximum displayed deviation and the actual deviation may be greater than the instrument can show.
Context Anchor
Seen while intercepting or tracking the localizer during an .