Cruising altitude is an altitude or flight level maintained during en route level flight. This is a constant altitude and should not be confused with a cruise clearance. In aviation, the term altitude can have several meanings and it is vitally important that parties exchanging altitude information are clear on which altitude definition is being used. In most countries cruise altitudes are often given in flight levels or feet and rarely given in meters.
The cruise altitude is maintained during cruise which is the level portion the aircraft travels where flight is most fuel efficient. Commercial aircraft are usually designed for optimum performance at their cruising altitude which varies by aircraft type/model, and conditions including payload weight, center of gravity, air temperature, humidity, and speed. Cruising altitudes are usually where higher ground speeds, increase in drag power, and decrease in engine power and efficiency are balanced at higher altitudes.
In addition cruising altitudes usually offer the aircraft better fuel economy as higher altitudes allow the aircraft to burn fuel more efficiently. Furthermore, aircraft often remain at their assigned cruising altitudes through their cruise to help expedite operational and air traffic control procedures.