The document defines and describes various terms related to ceilings, lighting, and fire protection in buildings. It includes definitions for terms like acoustic, which refers to the science of sounds; ceiling joist, which is the structural member that holds the ceiling board; and gypsum board, which is a board made of plaster with a paper covering. It also defines terms like insulation, which obstructs the passage of sound, heat, or cold; reflected ceiling plan, which is a drawing looking down at an interior ceiling; and sprinkler systems, which discharge water when effects of a fire are detected.