Homeostasis: The ability or tendency of an organism or cell to maintain internal equilibrium by adjusting its physiological processes.
Another way of saying this is that homeostasis is the complex set of operations that govern the organism's resistance to change. All living creatures use feedback loops to maintain their homeostasis. In an individual, when homeostasis fails, the organism becomes ill or dies.
As an example, in a human being, when the amount of fluid in the extracellular space begins to decrease below certain fairly specific limits (there is a dynamic range), various ions, chiefly sodium, increase in concentration. Through various routes which have not been fully elaborated, cells in the Hypothalamus react to this with a signal to the pituitary gland which then releases a hormone, anti-diuretic hormone (ADH), which not only causes the kidneys to more efficiently conserve water, but also signals the brain/mind that it is time to seek out some libations. Once the person drinks his fill, the sodium concentration returns to within normal limits and the sensation of thirst fades away. There is more to this of course, the stomach and mouth react, the blood vessels of the periphery are involved and many other things are affected as well, but this is essentially how thirst and fluid regulation work in people. If the dynamic equilibrium cannot be maintained, if the person cannot get anything to drink, the next steps in homeostasis would involve shutting down unnecessary bodily functions. The person would stop sweating, which can then impair temperature regulation. This is followed by more urgent emergency efforts to save electrolytes. The kidneys shut down completely. The "goal" is to maintain the osmotic balance in the brain as long as possible. Once the concentration of Sodium goes too high, the neurons in the brain can no longer function properly, some fire chaotically, leading to seizures, and when enough cells are damaged, death follows.
It is also possible to have too much water. Polydipsia is a condition most often seen in severely disturbed individuals (or occasionally in untreated diabetics) who drink upwards of 15 gallons of water a day. They can suffer from hyponatremia (low sodium), which can cause seizures and death. In this condition no ADH is released, the kidneys work at maximum output and can actually handle an impressive amount of fluid before the system breaks down.
Physiologically, there is a narrow band of acceptable concentrations of Sodium ions in the blood and the body does a very good job of regulating the concentration.
Non living systems can be thought of as influenced by homeostasis as well. (For those more inclined to a mechanistic view, homeostasis can be thought of as related to entropy versus order in systems. The amount of energy in a system will always tend to a minimum state as time goes on, unless the system is acted upon with a new source of energy. Systems thus tend to move toward lower energy states as time goes by.) The weather is an exceptionally complex system, filled with feedback loops that are not well understood, despite the arguments of the Global warming set. [They use computer models, which they admit are incomplete, to produce scenarios that are based on assumptions and very small sample sizes. They may be right, but for all anyone knows, the effect of CO2 on the global weather is swamped by the effects of increased water vapor (a much more powerful greenhouse gas) which may be more related to the sun's energy flux than anything we are doing. Caution is advised.]
For a simpler example, think of the recent tsunami: Where two large tectonic plates meet and crunch, there is a build up of energy which is typically released in small tremors over a long period of time. This is a stable structure. The periodic energy release allows the structure to remain stable in a dynamic system. When the energy becomes too great to release in measured amounts, as when the release is blocked for a long period of time, the energy builds until there is a catastrophic release. This is how the planet geologically manages the tendency to maintain stable energy states.
One thing that should be clear from describing both a human homeostatic mechanism and a geological one is that homeostasis is a deeply conservative principle. Homeostasis is a measure of the resistance of a system to perturbations (change); it doesn't get more conservative than that. Human beings are fundamentally conservative, as are all living things. People and other loving things adapt to changed conditions only when forced to do so; those that don't adapt, die.
What, you might be wondering, does this have to do with psychoanalysis and politics? Bear with me. I have pointed out how people are conservative in a physiological sense. However, people are also deeply conservative in a psychological sense. The human psyche is organized in a manner to maintain optimal functioning. This means that dysphoric states (depression, anxiety, panic, trauma) are to be avoided if at all possible and this involves, among other factors, maintaining a healthy self esteem and self regard. Sadly, self esteem had been misunderstood more than almost any other psychoanalytic concept. Some clarity is in order and will be supplied shortly.
To quote Garth Alger: "We fear change." (Dana Carvey)