A physiological adaptation is when an individual has a systemic response to an external stimulus, with the intent to remain in its homeostasis. Physiological adaptations differ from evolutionary adaptations because they don't involve transgenerational adjustment; instead, these are instead narrow in scope, having to do with an individual's change. A simple example of a physiological adaptation is when someone gets a tan from being out in the sun for a long time.
Other examples are when skin forms callouses from repeated contact or when organisms become adjusted to surviving in low-oxygen environments. Another definition for physiologic adaptation is when a cell undergoes an adjustment in response to a stimulus, allowing it to be better adjusted to its environment. This doesn't just apply to cells; tissues and various organisms can have physiological adaptations, as well. Plants can have physiological adaptations, too. Plants can accumulate water and self-prune. Furthermore, physiological adaptations don't have to be just physiologic in nature. They can also be metabolic.
Physiologic adaptations come in many different forms. They exist because they allow an organism to survive in its habitat. This is why some animals make venom, secrete slime, maintain a constant body temperature or purposely produce less urine. There are other kinds of adaptations, like behavioral adaptations and structural adaptations as well as adaptations that are determined by the environment.