Cruise altitude refers to the height at which an aircraft flies during the majority of its journey. This altitude is typically chosen for fuel efficiency, safety, and comfort. Commercial airliners usually cruise between 30,000 and 40,000 feet, where the air is thinner, reducing drag and allowing for smoother flight. Pilots determine the cruise altitude based on various factors, including the aircraft type, weight, weather conditions, and air traffic. Maintaining a stable cruise altitude helps ensure a consistent speed and allows for efficient navigation, making it an essential aspect of flight planning in aviation.