There are many synonyms for the term "one who is mentally ill". However, it is important to note that not all of these terms are appropriate or respectful. Some common synonyms include "mad", "crazy", and "insane", but these terms are outdated and can be offensive to people who are struggling with mental health issues. More appropriate synonyms might include "person with a mental illness", "individual with a psychiatric disorder", or "someone with a mental health condition". Using respectful language in discussing mental health is an important step in reducing the stigma surrounding mental illness and promoting acceptance and understanding.