Overview
A basic law is either a codified constitution, or in countries with uncodified constitutions, a law given to have constitution powers and effect. The term basic law is used in some places as an alternative to "constitution", implying it is a temporary but necessary measure, without the formal enactment of a constitution. The name is generally used to imply an interim or transitory nature, or to avoid claiming to be "the highest law", often for religious reasons.
West Germany
In West Germany the term "Basic Law" was used to indicate that the Basic Law was provisional until the ultimate reunification of Germany. However, when Germany finally reunified in 1990, no new constitutions were adopted and instead the Basic Law was adopted throughout the entire German territory.
Hong Kong and Macau
From Wikipedia (CC BY-SA 4.0).