Feedback is a process where part of the output of a system is measured and returned as input to regulate the systems further output.   Feedback normally involves gathering information on past performance from the output of a system, comparing it to a predetermined standard or plan and using any material deviations, as a basis of improving future performance. 

• Feedback can be negative (adverse) or positive (favourable)
• Feedback is based on comparing actual to a standard of performance
• Control action would be 'closing the stable door after the horse has bolted'