In plumbing, what does the term "drainage system" refer to?

The term "drainage system" specifically refers to a system designed for the removal of waste and wastewater from residential and commercial buildings. It is integral for maintaining sanitation and hygiene by transporting sewage, greywater, and other unwanted water away from living spaces to treatment facilities or sewer systems. This system ensures that pollutants are effectively managed to prevent backflow or contamination.

A system for water supply is not applicable here because it focuses on delivering clean water to fixtures rather than removing waste. While some systems might involve both supply and discharge elements, the primary function of a drainage system is exclusively concerned with waste removal. Additionally, a system for heating water relates to different plumbing functions, specifically concerning hot water supply and does not pertain to the movement of wastewater. Therefore, the key feature of the drainage system is its critical role in waste disposal, making the answer accurate in this context.