black letter

Short definition from Wiktionary

Noun

1. 1
   A highly calligraphic Western European script style used from approximately 1150: A Northern European style of type based on the script, with contrasting thick-and-thin, angular strokes forming upright letterforms, and usually set with a dark typographic colour on the page.
2. 2
   A highly calligraphic Western European script style used from approximately 1150: Text set in black-letter type.
3. 3
   The basic standard elements for a particular field of law, which are generally known and free from doubt or dispute.

Adjective

1. 1
   Written or printed in black letter.
2. 2
   Given to the study of books in black letter; old-fashioned.
3. 3
   Of or relating to the days in the calendar not marked with red letters as saints' days; unlucky; inauspicious.