Define "vertical speed indicator" (VSI).

The vertical speed indicator (VSI) is specifically designed to measure and display the rate of vertical movement of an aircraft, expressed in feet per minute or a similar unit. This instrument provides critical information to pilots regarding whether they are climbing, descending, or maintaining altitude. By understanding their vertical speed, pilots can make informed decisions about their flight path, ensuring safe and efficient transitions during ascent or descent.

In contrast to the other options, the VSI does not provide information about airspeed, altitude, or the pitch angle of the aircraft. Airspeed indicators measure how fast an aircraft is moving through the air, altitude indicators display the height of the aircraft above a reference point (usually sea level), and pitch angle instruments indicate the angle of the aircraft's nose relative to the horizon. Each of these instruments serves a distinct purpose in the cockpit, highlighting the specialized function of the VSI in monitoring vertical movement accurately.