Define the term "fixture unit" in plumbing.

The term "fixture unit" refers to a measure that reflects the probable demand for water from a plumbing fixture. This concept is crucial in plumbing design and system sizing as it allows plumbers and engineers to estimate the total demand of all fixtures in a system, considering factors such as flow rates and potential usage patterns. By using fixture units, designers can better understand the flow requirements and ensure that the plumbing system can adequately handle the expected water usage during peak demand times.

This measure is vital for effective system planning and helps in selecting appropriate pipe sizes, pump capacities, and other components of the plumbing system to ensure efficiency and functionality. The estimation is based on the characteristics and usage patterns of the fixtures rather than their physical size, cost, or pipe diameter, making it a focused approach to water demand analysis.