Feedback is where the output of a system is fed back into the input of the system in order to influence the overall result.
When you try to withdraw too much money from your account at a cashpoint a warning on the screen will advise you that it isn’t possible and will suggest that you try to withdraw a smaller amount. In this case, the input was your original request for cash, this was processed and the output would have been the money popping out. However, because there isn't enough money in the account, information is fed back to the cash point to let you know of the problem.
Another example is a microphone on stage that is too close to its speakers, you hear a nasty howling as the sound coming out of the speaker is fed back into the microphone.
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