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Mental breakdown (also known as nervous breakdown) is a non-medical term used to describe a sudden, acute attack of mental illness such as depression or anxiety.

Like the term "sanity," the terms "nervous breakdown" and "mental breakdown" do not have any medical definition and are not used in a clinical sense. A mental breakdown is not the same as a panic attack, though mental breakdowns can trigger panic.

Specific cases are usually described as a "breakdown" only after a person becomes unable to function in day-to-day life due to mental illness. The person's condition is then advanced, and seeking professional aid is likely advisable.