<unk>  is both a mechanism, process and signal that is looped back to control a system within itself.  This loop is called the<unk>  loop.  A control system usually has input and output to the system; when the output of the system is fed back into the system as part of its input, it is called the<unk> ."<unk>  and regulation are self related. The negative<unk>  helps to maintain stability in a system in spite of external changes. It is related to homeostasis. Positive<unk>  amplifies possibilities of divergences (evolution, change of goals); it is the condition to change, evolution, growth; it gives the system the ability to access new points of equilibrium.
For example, in an organism, most positive<unk>  provide for fast autoexcitation of elements of endocrine and nervous systems (in particular, in stress responses conditions) and play a key role in regulation of morphogenesis, growth, and development of organs, all processes which are in essence a rapid escape from the initial state.[citation needed] Homeostasis is especially visible in the nervous and endocrine systems when considered at organism level.
Feedback
Positive feedback
Measurement
Assay
Grading 
Therapy
Titration
Goal-directed therapy