Basically when you do something it gives you something back. In terms of the Internet, if I click on a button it will respond by sending information, or send me a file, or give me more information.

Not to denigrate Jimi Hendrix nor popular psychology, feedback is a concept in systems to maintain quality output. In an electrical circuit a small amount of the output is inverted and added back to the input. This cancels out portions of the noise created in the circuit. Why are electrical metaphors so popular?

The transmission of a signal from a later to an earlier stage. As a control mechanism, it acts on actual rather than expected performance. Cf. feedforward.

Negative feedback multiplies the input by a factor smaller than one (typically an exponential function of the difference between actual output and some ideal). Systems exhibiting this property thus tend to self-stabilize after disturbances. In biological systems, negative feedback is the basis of homeostasis.

Positive feedback involves a multiplier larger than one, and serves to amplify deviations. Such runaway systems are often brought under control by negative feedback; this usually results in the output following a sigmoidal growth curve.

Higher order feedback is feedback based on feedback. Second order negative feedback leads to sinusoidal oscillations about an equilibrium unless damped (by first order negative feedback).

Feedback may be continuous (eg. with an autopilot. Such a device is called a servomechanism) or discrete (eg. with a thermostat switching the heat on and off). It may also be intrinsic (the system itself adjusts its input according to its output) or extrinsic (the system's environment receives the system's output and thereby affects the system's input).

Feedback may be instantaneous or affected by delay (a pause until feedback occurs), lag (a gradual response to changes in feedback) or both. The latter three possibilities (and especially the latter) may cause negative feedback systems to become unstable, oscillating with ever-increasing violence, but may also allow a positive feedback system to restore equilibrium given appropriate conditions.

Nothing can happen in any system with any accuracy unless there is some feedback, and a sensor of some kind is used to provide that feedback.

In signal applications, devices such as those referenced by Wiccanpiper's excellent writeup in the detector node are also used as sensors to provide a reference to the control circuit of whatever system involved. The feedback is used as a reference to the original signal as a check to see how accurately the signal was carried through the system.

Feedback is vital to any control system. For example, in a power supply that drives a circuit, in order to provide proper current regulation there needs to be a feedback reference at the point of load to ensure that the current fed into the circuit maintians a sufficient level (just enough, not tooo much or not enough), feedback is provided backwards as a reference to tell the control circuit that the amount of current fed into the front end creates the proper amount of current at the back end.

In a robotic motion application, sensors provide the feedback that the nerves in a persons hand and finger provide to ensure that the pressure given is just the right amount. That is how a sophisticated robotic arm can pick up an egg. To simply set the robot's "hand" for a certain pressure isn't enough, there has to be a pressure feedback to compensate for differences in the thickness of the shell.

In an audio circuit, the feedback makes sure that the signal isn't over- or under-driven, and in the case of some subwoofers, there may be an actual physical motion sensor attached to the speaker cone itself to measure its movement (Velodyne makes a great example) to ensure that the signal that started makes it to the end exactly as intended (there is a school of audiophile thought that feels that excessive feedback makes the sound too sterile.)

However you slice it, unless you really, really know what is coming out of the other end you can never be in control of whatever system you create.

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