Unfortunately the exact definition of an Uninterrupted stay could not be found on an official site, so the following information is based on the Red Cross - Info Integration (Eupen) site.
The official sites refer to the local registry office, so asking there for confirmation that the text quoted below is correct is advised.
Since the application must be made locally, the local registry office will be checking their records to see if the residence requirements have been fulfilled.
Within the period of 5 years
- the sum of 12 months or
- a period of 6 months or more
Temporary visits outside the country do not count.
This is based on the Belgium Nationality law (BeStaGB) Art7bis (3), amended 01.01.2013.
Note:
The statement, from the unofficial site, that
- periods of absence must be only reported that are longer than 3 months
should be considered misleading, since Art7bis (3) BeStaGB makes no exception of the the sum of 12 month rule.
Ununterbrochener Aufenthalt:
Eine Unterbrechung des Aufenthalts liegt vor, wenn
- man länger als 6 Monate nicht in Belgien lebt (abwesend war) ODER
- die Abwesenheiten ein Total von mehr 1/5 des geforderten legalen Aufenthalts beträgt. Zum Beispiel legaler Aufenthalt von 5 Jahren: die Summe der Abwesenheiten darf nicht mehr als 12 Monate betragen.
In jedem Fall muss es sich immer um Abwesenheiten handeln, die der Gemeinde mitzuteilen sind (bei Abwesenheit länger als 3 Monate)
Uninterrupted stay:
There is an interruption of the stay if
- you live outside of Belgium for more than 6 months (were absent) OR
- the absences total more than 1/5 of the required legal stay. For example of a legal residence of 5 years: the sum of absences must not exceed 12 months.
In any case, absences must always be communicated to the municipality (for absences longer than 3 months)
Belgium Nationality law (BeStaGB), amended 01.01.2013
Art. 7bis
§ 3. In the cases provided for in this Code, the uninterrupted nature of the stay referred to in § 2 is not affected by temporary absences of up to six months, insofar as these absences total the duration of one fifth of the periods required in this Code for do not exceed the acquisition of nationality.
Sources: