Here is a quick and dirty definition per the FAA Part 121 Regs. There are a couple of alternatives this but it's not worth going into those under this particular discussion. I'm sure there are equivalent requirements under most regulatory organizations.
FUEL REQUIRED TO:
(1) To fly to and land at the airport to which it is released;
(2) After that, to fly for a period of 10 percent of the total time required to fly from the airport of departure to, and land at, the airport to which it was released;
(3) After that, to fly to and land at the most distant alternate airport specified in the flight release, if an alternate is required; and
(4) After that, to fly for 30 minutes at holding speed at 1,500 feet above the alternate airport (or the destination airport if no alternate is required) under standard temperature conditions.