The process by which organisms keep their internal conditions relatively stable is called homeostasis.
Although it is not officially a life function, without it, it would be very hard to survive.
The condition is constantly being threatened by changes in the environment, such as shifting temperatures and changing light. An example of homeostasis being carried out is when your breathing and heart rate automatically pick up in order to keep your cells supplied with enough oxygen when you're exercising, or when your pupils dilate in the dark to let more light in. Slowing down all body functions in cases of extreme cold is another example; in some animals this process can go so far as to become hibernation, and helps the animal to survive until food is plentiful again.
Did you mean homeostasis? Homostatisis is not in my vocabulary, nor is it in Merriam-Webster's. Homeostasis, however, is basically the body's way of keeping itself in a state of constancy, such as a constant or regular temperature. The body tries to keep itself at the same temperature (about 98.6 degrees Fahrenheit for humans) at all times. The process of attempting to maintain that temperature is homeostasis.