Define the term "crosswind."

The term "crosswind" refers to wind that blows across the runway, perpendicular to the length of the runway. This type of wind can significantly impact an aircraft during takeoff and landing because it challenges the pilot's ability to maintain straight flight along the runway's centerline. Managing a crosswind requires skillful control inputs from the pilot to counteract the forces acting on the aircraft, particularly during critical phases of flight when the aircraft is close to the ground.

Understanding crosswinds is crucial for pilots, as they must account for these conditions in their flight planning and execution. It requires specific techniques such as crabbing, where the aircraft is flown at an angle into the wind to counteract its effects, ensuring a safe and controlled landing or takeoff.

The other options do not accurately describe crosswinds, as they refer to conditions that either assist in flight or have other characteristics unrelated to the concept of a crosswind's direction relative to the runway.