This is a control term.
In any control system you have an 'input', 'process' and 'output'
But there is fourth item that may - or may not - be present. This item is called 'feedback'.
Feedback is a method of letting the input know what the output is doing. This means feedback can affect the way the control system works.
Feedback is often a good idea as it makes the system more accurate and responsive. The disadvantage is that it makes the design more complicated - it must be done right or the whole thing becomes unstable.
An 'unstable' system is where the output is swinging wildly from one value to another. For example if you place a microphone too close to the speakers of a music system, you will hear a horrible screeching noise - this is an 'unstable system' where there is too much positive feedback.
A control system is said to be 'closed loop' if it includes feedback
A control system is said to be 'open loop' of it does not contain feedback.
Challenge see if you can find out one extra fact on this topic that we haven't already told you
Click on this link: Feedback