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Here is the current term:  If below it different substantially compared to how it appears in your app/software, be sure to update your software/app's content prior to using this form.
Flight Level
A flight level (FL) is specific barometric pressure, expressed as a nominal altitude 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 true altitude either above mean sea level or above ground level.

Historically, altitude has been measured using a pressure altimeter, which is essentially a calibrated barometer. An altimeter measures air pressure, which decreases with increasing altitude following the barometric formula, and from the surrounding's pressure calculates and displays the corresponding altitude.

To display altitude above sea level, a pilot must recalibrate the altimeter according to the local air pressure at sea level, to take into account natural variation of pressure over time and in different regions. If this is not done, two aircraft could be flying at the same altitude even though their altimeters appear to show that they are at considerably different altitudes.

Flight levels solve this problem by defining altitudes based on a standard air pressure at sea-level. All aircraft operating on flight levels calibrate to this setting regardless of the actual sea level pressure.

Flight levels are described by a number, which is this nominal altitude ("pressure altitude") in feet, divided by 100, while being a multiple of 500 ft, therefore always ending on 0 or 5. Therefore an apparent altitude of, for example, 32,000 feet is referred to as "flight level 320". To avoid collisions between two aircraft due to their being at the same altitude, their "real" altitudes (compared to ground level, for example) are not important; it is the difference in altitudes that determines whether they might collide. This difference can be determined from the air pressure at each craft and does not require knowledge of the local air pressure on the ground.

Flight levels are usually designated in writing as FLxxx, where xxx is a one- to three-digit number indicating the pressure altitude in units of 100 feet. In radio communications, FL290 would be pronounced as "flight level two niner zero", in most jurisdictions. In some states, levels ending with 00 are read as "hundred": FL200 is pronounced as "flight level two hundred". The phrase "flight level" makes it clear that this refers to the standardized pressure altitude.

source: Wikitionary / Wikipedia and Related Sources (Edited)
Please do not use this form to tell us that the definition you saw doesn't necessarily match the context where you saw it. We use an automated keyword-based system to match the tens of thousands of definitions in our glossaries with our material. Sometimes, when a term has more than one meaning or use or when an abbreviation might stand for more than one thing or resemble a 'normal' word, this results in a definition for a term being shown that doesn't necessarily match the context. Please do not report such issues here. Rather, please use your best judgment to evaluate whether such definitions apply to what you're reading. If the answer is 'no' but nevertheless you feel the term doen't have a good definition in our system but should, please go back and find the 'suggest a term' link in the app. Thanks!
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