What does the term "fitting" refer to in the context of the Act?

In the context of the Act, the term "fitting" specifically refers to a component that is essential to the operation and safety of pressure equipment, such as a boiler. A gauge glass is a fitting that allows operators to visually assess the water level within a boiler. This is crucial for the safe management of the boiler, as maintaining the appropriate water level is vital to prevent overheating and potential failure of the equipment.

The other options represent components that, while related to the operation of a boiler or similar systems, do not specifically fall under the definition of "fitting" as outlined in the Act. A stack primarily serves as a chimney for exhaust gases, a fan is used for ventilation and air movement, and a feedwater regulator controls the water supply to the boiler but does not have the same direct role in indicating and maintaining the water level. Thus, the gauge glass, as a fitting, plays a critical part in the operational safety and efficiency of the system.