To stay alive, living things must constantly adjust to changes within themselves and to other changes in their environments. The ability of organisms to do this in another important sign of life called homeostasis.
For example, the optimum internal temperature for humans is 37oC (98.6oF). In response to the excessive hear of a very hot day, the body will sweat and the evaporation of perspiration will bring about cooling.
During colder times, some heat loss can be compensated for by shivering, which produces extra hear and warms the body.
Both responses are part of a homeostatic mechanism designed to maintain body temperature at its optimum value.
In each case, a group of specialized cells (the sensor) detected a change in temperature that was taking the body away from its optimum value of 37oC. These cells then triggered a course of action (the response) that is an attempt to return the body to the desired temperature.
Homeostatic mechanisms also participate in the replacement of skin and red blood cells, for example. Both are tissues that are constantly being replaced when old skin cells and blood cells are damaged or destroyed. The repair of wounds involves yet another type of homeostatic mechanism.
Homeostatic mechanisms maintain critical properties of a living organism at their optimum values or levels.
Constant sensing detects any change from the desired value. A response to a change is an immediate corrective action taken to adjust any drift away from the optimum settings. From these examples you can see that homeostasis is the ability of an organism to remain in the same state (at least on the inside) in the face of constant internal or external change.