Feedback occurs when outputs of a system are routed back as inputs as part of a chain of cause-and-effect that forms a circuit or loop. The system can then be said to feed back into itself. The notion of cause-and-effect has to be handled carefully when applied to feedback systems:

Simple causal reasoning about a feedback system is difficult because the first system influences the second and second system influences the first, leading to a circular argument. This makes reasoning based upon cause and effect tricky, and it is necessary to analyze the system as a whole. As provided by Webster, feedback in business is the transmission of evaluative or corrective information about an action, event, or process to the original or controlling source.

जब किसी निकाय में ऐसी व्यवस्था हो कि आउटपुट का एक भाग इनपुट में दिया जा रहा हो तो इस व्यवस्था का नाम पुनर्भरण या फीडबैक (feedback) है। पुनर्भरण एक उपयोगी संकल्पना है। स्वतः नियंत्रण में इसका अत्यधिक उपयोग होता है। पुनर्भरण मुख्यतः दो प्रकार का होता है- धनात्मक पुनर्भरण और ऋणात्मक पुनर्भरण।