Final answer:
A rough sketch used for initial discussions in set design is called a thumbnail, serving as a conceptual starting point before more detailed design work.
Step-by-step explanation:
A rough sketch that provides the basis for further discussions between the director and the scene designer is called a thumbnail. This preliminary sketch is often used early in the design process to understand design concepts and explore their strengths and weaknesses. A thumbnail is an essential starting point for visualizing and communicating initial ideas before moving on to more detailed and technical phases like creating mechanical scale drawings, models, and paint elevations.