Black letter law is the law that is well established and widely seen as uncontroversial. In some regions of the world, lawyers can also use this term to refer to the case law that has become widely accepted. In legal discussions, when people use the term, they generally do so with the implication that the law in question is accepted and not open to argument, unlike other types of laws that may be more open to interpretation.
This legal term is one of many that are often given false origin. People often say that it is linked to a famous American text, Black's Law Dictionary, but this is not really the case. The term "black letter law" was presented in Supreme Court decisions almost 50 years before this dictionary was published. The term is probably a reference to the type of black font, a font family which was often used for legal texts, even after it had fallen out of favor elsewhere print.
Other terms that people can use to fall well established, properly horn book law and trite law. "Hornbook law" is a reference to simple, basic, generally accepted law that people learn early in their legal training. In all cases the terms that are used when people talk about laws that are generally considered pretty clear and not vague in nature. Many of these laws come from the common law, with hundreds of years of history behind them, and people without legal training are often at least vaguely familiar with them.
Since lawyers often in doubt, challenge and reinterpret the law as part of their work, it is important to distinguish black letter law of other types of law. As expected, there is some debate in the legal community about whether a law should be considered truly undisputed. In general, the older a law and often it is mounted in court and in legal treatises, the more likely it is to be evaluated as generally accepted.
Changing societies call for changing laws and differing interpretations of existing legal precedent. This is crucial for the functioning of the legal system, because it does not stagnate to continue. However, black letter law is often at the basis of the law, and to support it is used in the arguments of the different interpretations of the law. A lawyer could argue, for example, that a reading of related black letter law supports his or her interpretation of a special status.