Plain English Meaning
The legal minimum fuel a pilot must have before takeoff: enough to reach the destination plus a reserve to keep flying for a set time after that.
Full Definition
The minimum amount of usable fuel a pilot must have on board before beginning a flight, as set by the Federal Aviation Regulations. Under 14 CFR 91.151, a VFR flight must carry enough fuel to fly to the first point of intended landing and then continue at normal cruising speed for at least 30 minutes during the day or 45 minutes at night. IFR flights have stricter requirements, including fuel to reach the destination, fly to an alternate airport when one is required, and then fly for an additional 45 minutes at normal cruising speed.