a title, heading, direction, or the like, in a manuscript, book, statute, etc., written or printed in red or otherwise distinguished from the rest of the text.

a direction for the conduct of divine service or the administration of the sacraments, inserted in liturgical books.

any established mode of conduct or procedure; protocol.

an explanatory comment; gloss.

a class or category

Archaic . red ocher.

written, inscribed in, or marked with or as with red; rubrical.

Archaic . red; ruddy.

noun

dictate , prescript , regulation