what is a ground loop?

I dont know.
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.
Thank you. I actually did search the archives, but the language used is sometimes difficult to sort through. Language that leads to other language- an abundance of all that, but still meaning is more difficult to come by. I appreciate your time.
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'.