What does it mean, "balanced?"

I'm new to theory of audio. Can anyone tell me what it means to be "balanced?" Someone told me it meant "zero feedback," which baffles me even more. Then what is "zero feedback?" The last question is about 220 V. I read that European 220 V is balanced. How does it affect stereo system compared to 110V in US? Thanks a lot in advance.
"Balanced" audio means the signal is split into its positive half and negative half (which are exact opposites of each other). Next, the two halves are compared and anything found in common is assumed to be external noise of some kind and is rejected. When the two halves are rejoined, there is nothing left except the pure original signal. This has nothing to do with feedback (which I don't know enough about to explain, sorry). Any feedback jockeys out there?
This is an area that is often misunderstood. Many, especially those within the audio community, have confused differential topologies with balanced. While many differential circuits may also be balanced, it is not necessarily true that all are. Balanced refers to the balancing of the common mode impedance. Balanced is not necessarily differential and therefore may not have opposite polarity signals that are centered around a common (such as ground). The beneficial property of a balanced circuit is in its ability to reject signals that are common mode in origin. The amount of rejection, referred to as CMRR (Common Mode Rejection Ratio) which is typically expressed in dB below the differential signal amplitude, can be used as a figure of merit when looking at the performance of a balanced circuit. Keep in mind that the CMRR achieved is function of the characteristics of both the transmitter and receiver. Kevin Halverson