Black letter law is the term that describes fundamental legal principles that are generally, but not necessarily universally, accepted without question. As such, it is applicable to widely recognized concepts in all areas of criminal and civil law. In the United States, it is also applicable to entrenched legal precedent.
The origins of the term "black letter law" are likely linked to the use of boldface for emphasis of important concepts in legal encyclopedias and related material.
In any case, a legal concept deemed black letter law is one that provides a definitive standard and can therefore be applied by rote. As such, its use seldom sparks argument or debate. Because black letter law effectively eliminates uncertainty or doubt, judges often rely on and cite these principles in their rulings.