This is a matter on which there is a difference of opinion (as is the matter of praying in the abnormal zones as a whole), and there is no single "right" definition; the simplest answer would be to pray according to the times of your local Islamic community, by whichever understanding they choose to follow.
The timings of the five daily salawat by the sun are well-known. In the abnormal zones, determining these times is based on being able to clearly delineate between them; when this is not possible, the nearest land in which they can be clearly delineated (and wherein there remains sufficient time to perform the salat) is to be used.
Abnormally long night
During the winter in the abnormal zones, one may have periods in which the sun does not rise (either partially or in full) for multiple days. The timings for maghrib, isha and fajr are easily determined by normal means, however given that both the dhuhr and asr salat are to be done in the latter half of the day (between mid-day and the beginning of sunset), one needs a clear period of time in which the sun is fully risen for long enough to perform both salawat.
Abnormally long day
On the opposite side of the spectrum, during the summer the abnormal zones may go for multiple days in which the sun does not set (either in full or in part); the inverse case thus applies. Calculating dhuhr and asr is simple, but one needs to be able to clearly delineate between maghrib, isha and fajr, all three of which are to be done after the sun has fully set. Again, the same basic principle applies wherein there needs to be enough time to perform each salat in its designated time. One point of dispute is whether the concession to combine maghrib and isha salawat should be taken into account:
- Combine prayers: One needs only a complete sunset and sunrise, between which sufficient time remains to perform the requisite salawat; in this case the combined maghrib/isha would be done between the time of sunset and midnight, and the fajr salat done between midnight and the beginning of sunrise.
- Do not combine prayers: One needs a period of time in which the isha salat is clearly defined, which typically means a long enough period of dark night (after twilight but before mid-night) in which to perform the salat. Some masajid in or near the abnormal zones alternatively consider isha to begin 90 minutes after the maghrib time starts, regardless of the appearance of twilight.