Plain English Meaning
The moment when the radio station you are tracking is directly out to the side of your aircraft — straight off one wingtip or the other.
Full Definition
In ADF (Automatic Direction Finder) orientation, the wingtip position is the relative bearing of 090° or 270° — meaning the NDB (non-directional beacon) station is directly off the right or left wingtip of the aircraft. It is used as a reference point for timing, intercepting bearings, and judging progress relative to a station.