Service gaps
Areas or times where people cannot reasonably reach what they need, because the service is absent, too far, or runs too rarely.
A service gap is a place, or a time, where people cannot reasonably reach something they need. In German, Versorgungslücken. The gap can be spatial, meaning nothing is within reach, or temporal, meaning the service exists but not when it is needed, which is the more commonly missed half.
Finding gaps is what most accessibility analysis is actually for. The method is the same each time: define what counts as reachable, in which time, by which mode, and for whom, then find the places that fail the test. The definition is the whole argument, because a threshold of fifteen minutes and one of thirty produce two different maps of the same region, and both are defensible.
The temporal half deserves its own attention. A bus stop with two departures a day passes a distance test and fails everything a resident cares about. This is why quality measures combine distance with frequency, and why an average across the day can hide a gap completely.