What is the maximum altitude associated with the term flight level?

Flight levels are a standard method of measuring altitude in aviation that corresponds to a specific set of atmospheric pressure. The term "flight level" specifically refers to the altitude of an aircraft expressed in hundreds of feet, measured from a standard reference pressure setting, which is generally 29.92 inches of mercury (Hg) at sea level.

In the United States, the transition altitude where aircraft switch from flying at altitudes (based on local barometric pressure) to flying at flight levels (based on a standardized pressure setting of 29.92 inHg) is 18,000 feet. Therefore, any altitude above 18,000 feet is referred to as a flight level.

As such, the definition clearly indicates that flight levels start at 18,000 feet and can reach significantly higher altitudes depending on the airspace and the specific operations of the aircraft. This makes "over 18,000 ft." the appropriate range associated with flight levels, distinguishing it from the lower altitudes which do not conform to this standardized setup.