Wiktionary
n. Any of several internationally agreed standard altitudes that are based on atmospheric pressure rather than on true height
Wikipedia
In aviation and aviation meteorology, a flight level (FL) is defined as a vertical altitude at standard pressure, nominally expressed in hundreds of feet. The pressure is computed assuming an International standard sea-level pressure datum of 1013.25 hPa (29.92 inHg), and therefore is not necessarily the same as the aircraft's actual altitude either above mean sea level or above ground level.
Usage examples of "flight level".
TWENTY-SIX CONDOR 10, FLIGHT LEVEL 650, 210 MILES EAST OF MYRTLE BEACH, SOUTH CAROLINA 7:28 P.
Come south, vector one-seven-zero, climb and maintain flight level three-three.
They watched green jagged lines that represented real mountains, ranks of them, fuzzy on the edges from the trees that grew to the tops of most, and for the most part the lines were well above the flight level of their aircraft until the last second, when the nose would jerk upwards and their stomachs would struggle to catch up, and then the aircraft would dive again.
There was as yet no instrument that detected turbulence, but up here at flight level three-niner-zero, that was a pretty rare occurrence, and Jackson wasn't often susceptible to airsickness, and his hand was inches from the yoke in case something unexpected happened.