Final answer:
The trap arm is the section of the fixture drain between the trap and its vent, crucial for carrying wastewater and preventing sewer gases from entering living spaces.
Step-by-step explanation:
The part of the fixture drain between the trap and its vent is known as the trap arm. The trap arm serves an important purpose in plumbing systems: it carries wastewater from the trap to the main drain, while ensuring that sewer gases do not enter the living spaces of a building. It's crucial that the trap arm is correctly installed and vented in order to maintain the trap seal, which is the barrier of water within the trap that prevents sewer gases from coming through.