On a window the casing is the molding or trim which generally covers the frame which holds the window assembly in place. The word “casing” refers to both the inside, which is normally more decorative and in keeping with the design of the home, and the exterior, which is typically less cosmetic but nevertheless a portion of the general exterior design of the home.
Chris Hill
The casing inside this illustration has a fluted look.
Kilner & Stohlman Remodeling Contractors
In odd spaces, the casing is equally important for finishing touches.
Smith & Vansant Architects PC
Uniform casing creates a uniform style for multiple windows in the exact same room.