In the context of mechanical systems, what signifies a pneumatic tool?

A pneumatic tool is characterized by its operation under fluid pressure, specifically the compression of air. These tools typically use compressed air to generate power, which makes them lightweight and efficient for a variety of applications in mechanical systems. This method of operation allows for high torque and versatility, enabling the execution of tasks such as drilling, grinding, and fastening without the bulk and weight of traditional electric motors.

The other options do not accurately define pneumatic tools. Electric tools rely on electrical power, hand-operated tools could refer to a variety of devices not limited to pneumatic systems, and hydraulic tools operate on liquid under pressure, rather than air. The unique operation based on air pressure is what distinctly classifies a tool as pneumatic, setting it apart from these other categories.