pelmet (cornice)

In British English, “pelmet” covers both rigid structures (like cornices) and soft treatments (like valances). Americans typically say “cornice” for the hard box and “valance” for the soft fabric. 

Synonyms:

• Cornice or Cornice Board: The rigid, structured box
• Valance: The soft, fabric treatment
• Window Topper: General term for anything decorative above windows
• Box Valance: Sometimes used for fabric-covered rigid structures

A cornice is that rigid, box-like wooden frame, usually wrapped in fabric or painted, sitting above a window and its curtain/drapery. 

Cornices provide surprising insulation benefits. The rigid box creates dead air space above the window. Cold air stays trapped instead of flowing into the room.

A valance takes the soft approach entirely. Pure fabric hanging loosely across the window top.

source: https://www.blindsgalore.com/blog/index.php/blinds-basics/difference-between-cornice-vs-valance-vs-pelmet