Adaptive reaction, also known as adaptive response, is the appropriate reaction to environmental stimulus.
For instance, the iris of the eye expands and contracts in response to the brightness level in the environment. This is an automatic response of the eye, rather than a conscious reaction. Likewise, humans and animals have the adaptive reactions of "fight or flight" when encountering a new, frightening or dangerous situation. This reaction causes the release of adrenaline into the body preparing it to either fight or run away from a situation.
Likewise, the body has many built-in systems that subconsciously prepare the body or mind for routine activities. For instance, when a person who is in the habit of having an alcoholic drink every evening their body will automatically prepare itself for an infusion of alcohol. When a person smells food it is natural for the saliva glands to salivate and prepare the mouth and body for food consumption. These are just a few of the many subconscious adjustments that the body makes foe routine activities.