Which term is used to describe a decline in physical or mental health?

The term that reflects a decline in physical or mental health is illness. This term is broadly used in health care to describe a state in which an individual’s normal physiological or psychological functions are impaired. It encompasses a wide range of health conditions, including both acute and chronic diseases, and signifies that a person is experiencing a deviation from a healthy state, whether that be through physical ailments, mental health disorders, or a combination of factors.

In contrast, wellness denotes a state of health that includes physical, mental, and social well-being—essentially the opposite of illness. Stability refers to a state of being steady or unchanged, not specifically related to health declining or improving. Recovery signifies the process of regaining health or returning to a previous level of functioning after an illness, which further illustrates that the term is concerned with a positive return to wellness rather than a decline. Therefore, illness is the appropriate choice to describe a decline in health.