A holding pattern is a particular flight maneuver. In controlled airspace operations, it is sometimes necessary for an aircraft to wait its turn, perhaps because the runway or airport for which it is bound is busy. Perhaps several aircraft are attempting to enter a flight corridor, and the air traffic controller needs to have them wait and enter the corridor with a specified separation.
Of course, airplanes can't just stop. Nor is it practical for them to land for the minutes required. As a result, they must continue to fly, but remain in a predictable and narrow range of airspace. Enter the holding pattern. Typically, a holding pattern is based on a specific location called a holding fix. This is usually some form of landmark, either a visual landmark or a navigation beacon of some sort. Modern aircraft can use GPS coordinates as a fix. In any case, the holding pattern itself is generally a racetrack path flown with the aircraft approaching and then passing over the fix point immediately before beginning one of the two 'end' turns of the racetrack to 'enter' the pattern:
fix
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--------------> O --
/ \
/ \
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\ /
\ /
-----------<-------
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flight path
Holding patterns are an integral part of IFR flight, and IFR pilots must be able to fly them predictably and regularly so as to ensure that controllers can properly arrange IFR aircraft when holds are required. Generally, the pattern itself is four minutes of flight - one minute in each 180 degree turn, and one minute of straight flight along each edge. Transport pilots prefer larger patterns, as frequent turns can be disturbing to passengers. The holding pattern is usually flown at slower than normal cruise speeds, both to minimize the amount of airspace the pattern requires and to minimize the amount of fuel the aircraft uses while 'in hold.' One advantage of the holding pattern is that multiple aircraft can be directed to fly the pattern at different altitudes. This is termed a 'holding stack'.
IroN Noder 2010