昏厥(syncope)是一種暫時性的意識喪失(transient loss of consciousness)，身體無法維持姿勢的張力，通常是由於腦部血流灌流不足(cerebral hypo-perfusion)引起，病人通常不須經由任何處置，會自行完全恢復意識。昏厥前的症狀並不具特異性，可能會有頭痛、頭暈、噁心、冒汗、無力或視力模糊等，而腦部因突然血流不足，失去意識，通常發作時間約10-20秒，很少超過數分鐘。昏厥的原因很多樣化，常見原因包括：（1）反射調節性昏厥；（2）起立性昏厥；（3）心因性昏厥。昏厥可由多重因素造成，大部分的昏厥起因於反射調節性昏厥，仔細的病史和身體理學檢查常可找出大部分的病因，診斷確立後，昏厥的病患則需針對病因來加以治療，大部分病人預後良好。為了預防猝死或意外傷害，早期診斷出心臟結構及血管異常乃是必要的。

Syncope is defined as a transient loss of consciousness (TLOC) with a loss of postural tone, usually caused by cerebral hypo-perfusion. Patients with syncope usually can restore consciousness spontaneously without specific treatment. The prodromes for syncope are non-specific, and may include headache, dizziness, nausea, sweating, weakness and blurred vision. The TLOC might last for 10-20 seconds, and rarely exceed for minutes. The causes of syncope are very diverse, and common causes may involve: (1) Reflex syncope; (2) Syncope due to orthostatic hypotension and; (3) Cardiac syncope. The causes of syncope are multi-factorial, and reflex syncope constitutes the majority of clinical syncope. Careful history taking and physical examination can identify common etiologies for syncope. Once the diagnosis for syncope is established, patients should be treated according to their specific causes. The prognosis for most patients is good. However, it is necessary to identify abnormalities in cardiac and vascular structures in order to prevent sudden cardiac arrest or accidental events.