The medical term for an unknown cause of a heart attack is "idiopathic myocardial infarction." This term is used when a heart attack occurs without an obvious underlying cause, such as coronary artery disease or other known risk factors. The diagnosis of idiopathic myocardial infarction is made after other potential causes have been ruled out through testing and examination. It is important to note that even in cases of idiopathic myocardial infarction, lifestyle changes and medical treatment are still necessary to prevent future cardiac events.