Mental breakdown is a non-medical term used to describe an acute, time-limited phase of a specific disorder that presents primarily with features of depression

Clinical depression

Major depressive disorder is a mental disorder characterized by an all-encompassing low mood accompanied by low self-esteem, and by loss of interest or pleasure in normally enjoyable activities...

 or anxiety

Anxiety disorder

Anxiety disorder is a blanket term covering several different forms of abnormal and pathological fear and anxiety. Conditions now considered anxiety disorders only came under the aegis of psychiatry at the end of the 19th century. Gelder, Mayou & Geddes explains that anxiety disorders are...

.
The terms "nervous breakdown" and "mental breakdown" have not been formally defined through a diagnostic system such as the DSM-IV or ICD-10

ICD-10

The International Statistical Classification of Diseases and Related Health Problems, 10th Revision is a medical classification list for the coding of diseases, signs and symptoms, abnormal findings, complaints, social circumstances, and external causes of injury or diseases, as maintained by the...

, and are nearly absent from current scientific literature regarding mental illness. Although "nervous breakdown" does not necessarily have a rigorous or static definition, surveys of laypersons suggest that the term refers to a specific acute time-limited reactive disorder, involving symptoms such as anxiety or depression, usually precipitated by external stressors.

Specific cases are sometimes described as a "breakdown" only after a person becomes unable to function in day-to-day life due to difficulties adapting.
Causes of such breakdowns are varied.