Plain English Meaning

Stretches along a route where the ground-based navigation signal drops out for a short time. The gap is allowed only if it's small enough that you can fly through it safely before picking up the next signal.

Full Definition

Segments of airspace along an airway or route where signal coverage from ground-based navigation aids (such as VORs) is unavailable or unusable, leaving the pilot temporarily without that source of electronic course guidance. The FAA evaluates these gaps and may permit them when they are short enough — based on aircraft speed and altitude — that the aircraft can cross them without losing safe navigation continuity, especially when other means (GPS, DME, radar) are available.