noun 1. the returning of a part of the output of any system, especially a mechanical, electronic, or biological one, as input, especially for correction or control purposes, to alter the characteristic sound of conventional musical instruments, etc.The Macquarie Dictionary Online © 2007 Macquarie University, Published by Macquarie Dictionary Publishers Pty Ltd.
2. an indication of the reaction of the recipient, as of an audience.
3. Electronics the return of part of the energy of the output circuit of an amplifier to the input, either to oppose the input signal (negative feedback) or to reinforce it (positive feedback).
4. the input of a signal into a microphone from the output of the same system, usually causing a high-pitched screech.
1 is the most general, including the deliberate special case 3 and the unintentional 4. 2 can also be a case of 1, but are its effects sometimes more like 4?