Plain English Meaning
An imaginary line running outward from an NDB ground station. When the aircraft flies across that line, the ADF needle shows it, and the pilot knows they are at a known point along their route.
Full Definition
A bearing line from a non-directional beacon (NDB) that intersects an aircraft's course or flight path, used to identify a specific point along that course when the primary fix-identifying equipment is inoperative or unavailable. The pilot determines position by noting when the relative bearing to the NDB indicates the aircraft is crossing the published bearing line.