Biofeedback is a scientific technique aiming at training you how to desensitize from factors that cause you stress or anxiety, to gain control over certain functions in your organism and thus exploit your inherent potential and be released from health problems, in a manner indeed completely governed by you and your goals.
Or more descriptively:

Once you become cognizant, assisted by a device, of an internal function in your body, of which you were unaware up to that moment, you get the chance to learn how to control and modify this function to a great extent, with the purpose of reaching optimal competence and performance both at internal homeostasis and external activity. This entire process is called biofeedback.

Biofeedback, as a natural and physiological function, is meant for all of us. Viewed from the angle of the offered benefits biofeedback concerns those who deem as essential to exploit their entire potential and be freed from stress and anxiety.

Granted that every biofeedback protocol is based upon your potentialities and needs, it essentially represents a tailored suit. How well it fits depends on the tailor, your trainer. But let’s treat the issue in a more exhaustive manner.
The definitions and analyses referred above made clear that practical and applied biofeedback is divided into two big sections, depending on whether it serves a therapeutic or regulatory purposes. The first great category refers to therapeutic or clinical biofeedback and the second refers to regulatory and preventive biofeedback.
Τhe first category of biofeedback applications encompasses the therapeutic treatment of illnesses and syndromes, whereas the second engages in the regulation of the organism, enabling us to normalize some of our physiologal functions, leading so to disease prevention and or to optimal performances, therefore being multiply beneficial. Often, biofeedback application is mixed and everything depends upon the needs of your organism.
Your trainer or therapist will evaluate your needs and suggest the suitable program.
You, therefore, decide on what category you think you belong.

-Are you a person presenting some torturing psychosomatic symptoms, wishing to get rid of them?
-Are you a stressed and anxious modern person, realizing that you are on the brink of your resistance and in need of regulations in order to endure?
-Are you a college or university student with high aspirations, who has to read heaps of books in a short time to avoid loosing the semester?
-Are you an administrative executive in a firm, almost about to loose your job from your rage explosions?
-Are you a student? Do your future studies seem like a tall order to you? Is anxiety nailing you down, do you think that no matter how hard you try you will accomplish nothing and thus quit trying?
-Are you a highly promising athlete who aims at peak performances?
-Are you a journalist living in the frenzy of everyday life, news and deadlines?
-Are you an actor who prefers to get established to the broader public or to an audience circle that selects you? Since you can in a one circle, why not in every other as well?
-Are you an administrative director loaded with many responsibilities, do you have to settle successfully a heap of abeyances, to deal and smooth out complains and conflicts?
-Are you a driver in town and must always be “alert”?
-Are you a woman with two school age kids, who works, wakes up at sunrise or stays up late at night to cook, who returns from work and must study with the kids, play the “taxi” to and from their out-of-school activities, who must have a thousand hands?
-Are you a career woman, who works around the clock to make both ends meet who finds that there is never enough time to arrange both your works and take good care of yourself?
-Are you in the trade or in business struggling everyday with expiring cheques, living the market frenzy and competition, having to organize and supervise your stuff?
-Are you a manager, insurer, administrative executive who must be psychomentally flexible every moment so as to use in the best possible way your potential?
-Are you a salesman full with anxiety about your presentation and its outcome?
-Are you in the teaching profession and have to deal daily with those “sweet monsters”, and i “win them over”, inspire and drive them to the proper learning and moral direction? Does this task seem to you a bit tough in our age?
-Are you a scientist working under harsh conditions with payment, however, incommensurate to your work, a burden which you cannot unload?
-Are you an intern or hospital doctor working exhausting time tables, facing desperate relatives or emergencies which call for all your body and soul reserves, and demand instant composed action?
-Are you in private practice overwhelmed with the survival anxiety or the desire move up in rank in your professional field?
-Are you an employee and everybody are pressing? Does your boss too take it out on you?
Are you working smoothly in your job, but feel “stuck in the groove” and this “gets on your nerves”?
-Are you participating in public affairs? Besides feeling stressed from your unavoidable public exposure, do you also fret on how you will survive and prevail over rumors, intrigues and nastiness?
-Are you a judge who must rule imperturbably and uninfluenced from circumstances? Does this fact raise inside you a lot of stress?
-Are you an attorney, fighting daily with adversities?
-Are you a person without occupational, family or financial problems, everything is working fine for you, however you worry too much about your health and you wish to strengthen it?

If you identify with one of the above then biofeedback is suitable for you.

The science of biofeedback was born seventy years ago. Its roots are stretching in the principles of learning, in the field of experimental psychology and neurology.

Long back, before it was established as organized science, various scientists had started to study solitarily the interaction between bodily and psychological functions. In the 1780’s Luigi Galvani had already figured out the important role electric energy played in the muscle movement. He proved with his experiments that every muscular contraction is accompanied by an electric change of muscular activity, which can be traced by a device, studied and recorded.
In 1890, the Italian criminologist Cesare Lombroso started measuring the blood pressure of suspects interrogated by the police. He was the first to suggest that blood pressure could prove useful in monitoring the psychological processes taking place in a person.
In 1930, Edmund Jacobson used an electromyograph to investigate the objective effect of progressive release of chronic accumulated muscular tension upon the level of muscle activity.
Systematic research in the field of biofeedback commenced in the research laboratories of mid ‘30. Around 1940 Ε.Green, H.D. Kimmel, Neal Miller and David Shapiro were among the first psychologists who treated the issue systematically. Biofeedback was initially used to explore animal and human capacity to control the progress of certain functions of the organism, functions which up to that time were considered that could not be placed under voluntary control. The prevailing opinion at that time was that, for instance, gastro-intestinal, cardiac, thermoregulatory functions were totally involuntary. Those early pioneers, hypothesized that if man could gain voluntary control over these functions, as well as others, it would open tremendous perspectives for the treatment of several diseases. Those pioneers suggested that a person could learn to exert direct control over vasoconstriction (vessel constriction) and  prove helpful in the treatment of vasomotor type migraines. The continuous increase of biofeedback’s applications in the control of many physiological functions, eventually led to the successful curative interventions in numerous disorders.
The term biofeedback is relatively new in medical science and means providing the organism with information relating to a biologic signal the organism generated. The term feedback in mechanistic systems indicates a mechanism which receives information on the operation of a mechanistic system, then feeds back this system with information and, usually, gives also an order. For example, a system of this type is the organ measuring the temperature of car coolant. It receives constantly information about this modality and as soon as the measurement exceeds a certain limit, it orders the thermostat to open and circulate the refrigerant in the system. This means that it receives feedback from the environment of the car and gives instructions according to its pre-set regulations.
Similarly, the term feedback is used when we discuss human organic systems or modern robotic systems which “duplicate” human functions. However, in this case information feedback does not imply the typical direct order, but the alteration of a function through voluntary behavioral change.
The term “feedback” was invented by the mathematician Norbert Weiner, who defined it as “a control method of a system achieved by re-entering to the system the results or outcome of its previous performance” (Βirk, 1973). The physiological information fed back can be either realized or not. In either case it is information fed back to the system.
Neal Miller, one of the scientists we mentioned previously, instrumental in the development of biofeedback gives a descriptive definition:

“Biofeedback is the use of modern devices to provide a person with better moment to moment flow of information regarding a certain physiological function which, notwithstanding being under the control of the nervous system, was not  before easily or at all perceived. In servo-systems terminology this information is called feedback. However, when this information refers to biological functions it is called biofeedback”.

Τhere are many definitions and descriptions of biofeedback. Those that define biofeedback with respect to the process adopted and those that define it based on the pursued goal.
In connection to its modus operandi biofeedback could be described as follows:
1. Swartz & Beatty, in 1977, defined: “Biofeedback is a term recently invented which refers to a group of experimental processes, where external stimulus is used to supply the organism with indications, or a picture of the condition of a bodily function. This entire process, usually, takes place during the effort to obtain changes in the measured parameter of the monitored function”.
2. Gaarder & Montgomery, in 1977, specified: “The term biofeedback is mainly used to describe a process. A more accurate term would be external psychophysiological feedback”.
3. Κamiya, in 1971, said about biofeedback application: “Initially, the physiological function we wish to control must be monitored with great precision, so that the occurring changes can be followed moment to moment. The alterations of the psychophysiological modalities must reflect directly upon the trainee who endeavours to bring his function under his control. The person must learn how to affect the physiological changes he monitors”.
With regard to the pursued objectives, biofeedback could be defined in the following terms:
1. Οι Ray, Raczynski, Rogers & Kimball, in 1979, defined:

“The basic and primary goal of biofeedback is to promote and support the self-control of the individual over its physiological functions”.

2. Brown, in 1977, defined: “Biofeedback is the process or technique via which one learns to voluntarily and automatically control the reflexes of the somatic functions he wishes to regulate”.
3. Green, in 1977, defined:

“Biofeedback training is a tool assisting a person to learn to exert psychosomatic self-regulation”.

Let’s see some more general definitions:
1. Birk, in 1973, defined: “As biofeedback can be defined the method employing devices, usually electronic, which detect and enhance bodily functions, in such a manner that deeper information regarding these functions, usually unavailable consciously, become accessible to the trainee and are fed back to him in the form of a reading”.
2. Hassett, in 1978, defined:

“Biofeedback is the process during which the trainee becomes aware of the subtle changes in one’s physiological functions, aiming at realizing the underlying mechanism and bringing them under voluntary control”.

3. Basmajian, in 1979, stated: “Biofeedback can be defined as the technique usually employing electronic equipment to reveal man some of the physiological or pathological incidents taking place inside him. The revelation takes the form of audio or visual signals emitted by the devices, enabling a person to handle these incidents, which without the mechanical aid would remain imperceptible. Learning how to handle them is achieved by elaboration of the signals produced by the devices”.
4. Schwartz & Fehni, in 1982, defined: “Biofeedback uses sensitive electric or electro-mechanic devices in order to measure, process and feedback the ongoing activity of various bodily functions, which the person usually ignores. Through this procedure the patient or trainee has to opportunity to alter his somatic functions or gain beneficial control”.
Conclusively, my definition for biofeedback includes:

Biofeedback is a scientific technique aiming at training you how to desensitize from factors that cause you stress or anxiety, to gain control over certain functions in your organism and thus exploit your inherent potential and be released from health problems, in a manner indeed completely governed by you and your goals.
Or more descriptively:
Once you become cognizant, assisted by a device, of an internal function in your body, of which you were unaware up to that moment, you get the chance to learn how to control and modify this function to a great extent, with the purpose of reaching optimal competence and performance both at internal homeostasis and external activity. This entire process is called biofeedback.