A basic, settled tenet of law, notorious and well known.
In cases, judges often premise their judgments by statements that a certain principle is "black letter law"; that it is well-known and ought not to be in dispute.
Judges in some jurisdictions, such as Canada or England, prefer the expression that a specified principle is trite law, as in "it is trite law that (a court sitting on judicial review) cannot cannot consider evidence not available to the decision-maker" (Singh v Canada 1997 CanLII 5208, Federal Court of Canada).
Some examples include Justice Aldrich of the US Court of Appeals, 3rd Circuit, in Donald Manter v Davis (543 F 2d 419, 1976):
"It has long been black letter law that personal service within its geographical area establishes a court's personal jurisdiction over the defendant."
Or in Beaulieu v US (865 F 2d 1351, 1989):
"It is black letter law that it is a party's first obligation to seek any relief that might fairly have been thought available in the district court before seeking it on appeal."
Finally, Miller v US Postal Service (825 F 2d 62, 1987):
"It is black letter law that res judicata ... bars all claims that were or could have been advanced in support of the cause of action on the occasion of its former adjudication, ... not merely those that were adjudicated."