In plumbing, what does the term 'liquid seal' refer to for fixture traps?

The term 'liquid seal' in plumbing specifically refers to the amount of water that is retained in a fixture trap. This water serves an essential function by creating a barrier that prevents sewer gases from rising back up into the living space. The liquid seal must be adequately maintained to ensure the trap functions effectively, helping to protect indoor air quality.

While other aspects related to traps, such as the distance between the fixture and vent, the material of the trap, and the drain pipe diameter, play important roles in plumbing systems, they do not directly define the concept of a 'liquid seal.' Thus, the correct focus on the definition of the liquid seal as the water level within the trap emphasizes its vital role in plumbing fixture functionality and hygiene.