Glossary of Literary Theory by Greig E. Henderson and Christopher Brown

Decorum:

A term used by classical, Renaissance, and neoclassical critics (Horace, Sir Philip Sidney, John Dryden, and others) to describe the mutual appropriateness of genre, style, action, subject matter, and character. For example, a high style is fit and proper for royalty, a grave style for old men, a rustic style for shepherds, and a prosaic style for clowns. According to the dictates of correctness and good taste, the genre (tragedy, comedy, epic, or another), style (high, middle, low), action (whether serious or comic), subject matter (death, marriage, and so on), and character (age, rank, and status) must decorously merge. (See also Art, Delight, Instruction, Nature.)