Feedback mechanism. Definition. noun. (1) A loop system in which the system responds to perturbation either in the same direction (positive feedback) or in the opposite direction (negative feedback).(2) A process in which the level of one substance influences the level of another substance.(3) A mechanism or a signal that tends to initiate (or accelerate) or to inhibit (or slow down) a process. Supplement. An example of positive feedback loop is the onset of contractions in childbirth.2) A process in which the level of one substance influences the level of another substance. (3) A mechanism or a signal that tends to initiate (or accelerate) or to inhibit (or slow down) a process. Supplement. An example of positive feedback loop is the onset of contractions in childbirth.