What does "Flight Level" mean?

The term "Flight Level" specifically refers to an altitude that is expressed in hundreds of feet above sea level. This is a standardized way of communicating altitude to ensure clarity and safety between aircraft and air traffic control. Flight levels are typically used when an aircraft is operating at high altitudes, and they are prefixed by the letter "FL" followed by a number, such as FL350 for 35,000 feet. This system is essential in aviation as it helps maintain separation between aircraft flying at different altitudes, particularly in controlled airspace.

The other options refer to different concepts: speed of the aircraft pertains to ground speed or airspeed; altitude in meters is a metric measure often used outside of the U.S.; and fuel efficiency relates to how well an aircraft uses fuel, which is not the same as indicating an aircraft's altitude.