The name or heading under which something is explained;
an explanation or a set of instructions at the beginning of a book or test, etc;
an established rule, tradition or custom.
1) The rubrics at the beginning of the chapters are quite humorous.
2) The rubric, at least among jewellers, is that a man should spend at least a month of his salary on his fiancée's engagement ring.
Middle English rubrike which means 'red orcher', heading in red letters of part of a book, from Anglo-French, from Latin rubrica, from rubr-, which means 'rubber red'.
caption, head, header, headline, heading, title