What action describes "descent" in an aircraft operation?

Descent in aircraft operation refers specifically to the phase where the aircraft is transitioning from its cruising altitude to a lower altitude. This phase is crucial during flight as it involves systematic and controlled decrease in altitude, preparing the aircraft for landing. During descent, pilots manage various factors such as airspeed, descent rate, and altimeter settings to ensure a safe approach and landing.

The other choices focus on different aspects of aircraft operation: landing preparation pertains to the steps taken before landing, gaining altitude describes the process of climbing after takeoff, and taxiing refers to the movement of the aircraft on the ground, either on the runway or taxiways. Each of these actions occurs at different phases of flight and does not accurately represent the specific concept of descent.