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In an airplane, it's a maneuver you do when your brakes fail and you are about to go off the end of the runway. You put a wingtip in contact with the ground and the plane spins out and stops quite quickly with little damage.
In audio, it refers to the circuitry being grounded at more than one place in the circuit...say at the phono preamp and again at the power amp. The circuit and the two grounds form a closed loop. Because the ground potential is probably different at the two (or more) places that are grounded the result will be hum.
To Kr4--Searching here probably would be more useful.
Tim, a ground loop is a situation in which electical components in one system get plugged into electrical outlets with grounds (neutrals/commons) unintentionally at different zero voltages. Hence the different chassis are at different 'zero' voltages (all of which SHOULD be zero but aren't). Current then flows thru the common legs of interconnect cables and induces 60- or 120-cycle hum into the signal. It happens quite often in systems with many pieces of equipment (such as mine, damn it!), and it's most-easily patched (but not solved) by floating (detaching) the offending equipment's third-wire ground. I said 'patched' above because while the system's most-obvious hum may be mostly gone, there's still low-level garbage deteriorating to some extant the sound quality.
The real solution is to have one's house or at least room rewired by someone who knows how to bring EVERY outlet in the room to the same 'ground'.
Jeffreybehr - Don't fire your electrician yet. Even with great house wiring you can still get induced ground current inside your equipment and have ground loop problems. E.G., I think Jim Hagerman might have some info about a star ground configuration he used on his trumpet phono pre-amp to try and minimize the problem (WWW.Hagtec.com, or something like that). I think your comment about getting the house wiring right is on the money, but good equipment internal design (and/or a little luck)is also part of the issue.