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 the cruise, which is the level portion of the aircraft's travels where flight is the 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 typically where higher ground speeds, increase in drag power, and decrease in engine power and efficiency are balanced at higher altitudes.
In addition, cruising altitudes typically offer the aircraft better fuel economy, as higher altitudes allow the aircraft to burn fuel more efficiently. Furthermore, aircraft regularly remain at their assigned cruising altitudes throughout their cruise to help expedite operational and air traffic control procedures.