In hydraulic systems, what does the term 'hydraulic fluid' usually refer to?

Hydraulic fluid typically refers to oil in hydraulic systems. This choice is correct because hydraulic systems rely on fluids to transmit power. Oil is preferred for several reasons, including its lubricating properties, high density, and ability to maintain consistent pressure.

Using oil as a hydraulic fluid ensures effective energy transfer within the system while minimizing wear and tear on components. It also has a higher viscosity compared to other liquids, which helps in maintaining better performance under varying temperature conditions. Additionally, oil can help in preventing corrosion and can provide a seal against contaminants, making it ideal for the demanding environment of hydraulic systems.

In contrast, gas or air would not provide the necessary incompressibility required for proper hydraulic function, and water, while sometimes used in specific applications, does not offer the same protective and lubricative properties as oil. Thus, oil is the standard hydraulic fluid in most systems.