Final answer:
Mental Illness refers to all mental disorders with a definable diagnosis where a person's thinking, mood, or relating with other people may be impaired.
Step-by-step explanation:
Mental illness refers to a range of mental disorders with a definable diagnosis. These nervous system disorders result in problems with thinking, mood, or relating with other people, and can be severe enough to impair a person's ability to perform everyday tasks.
There are various mental disorders, including schizophrenia, major depression, bipolar disorder, anxiety disorders and phobias, post-traumatic stress disorder (PTSD), and obsessive-compulsive disorder (OCD). The American Psychiatric Association's Diagnostic and Statistical Manual of Mental Disorders (DSM) is the primary reference for diagnosing these conditions.
In the context of the question, the blank could be filled with 'thinking, mood or social functioning' as these are key areas affected by mental illnesses.