What term describes the fabric that designates the acting area or part of a set?

The term that describes the fabric designating the acting area or part of a set is "floorcloth." A floorcloth is typically a large piece of fabric, often painted or decorated, that covers the stage floor. It can serve both practical and aesthetic purposes, helping to define the acting space and contributing to the visual design of the overall set.

In theatrical productions, floorcloths can be used to create various environments and settings, providing a textured surface that adds depth to the visual experience for the audience. The choice of colors and patterns on a floorcloth can enhance the mood and tone of the performance.

The other terms listed do not refer specifically to the fabric that outlines the acting area. "Frontcloth" generally refers to a curtain or drop that hangs at the front of the stage, often used for masking or transitions. "Donut" is not a standard term in set design or stage architecture, while "douser" typically refers to a device used for extinguishing stage lights rather than any fabric related to acting areas.