昏厥为快速发生、暂时性的、可自发性回复的意识及肌肉张力丧失之状态。昏厥起因为血液供应大脑的灌流不足，且通常由低血压引起。某些原因引起的昏厥在失去意识前会有前驱症状（英文：prodrome），包括头晕、盗汗、皮肤苍白、视力模糊、恶心、呕吐、心跳加速以及异常温暖的感觉等。昏厥也可能伴随着短暂的肌肉抽搐。昏厥前期（英文：presyncope）指的是类似症状，但尚未完全丧失意识及肌肉张力的状态。根据目前建议，不论是昏厥或昏厥前期的病人其医疗处置都是一样的

会导致患者引发昏厥的原因众多，包含了轻微的疾病，亦包括有可能危及生命的重症，可大致分为三类：心血管相关、反射或神经系统调控引起的昏厥、姿势性低血压。心血管部分虽仅占10%，却最严重；而神经系统调控引起的昏厥则为最为常见。心脏相关引起的昏厥包括心律不整、心脏瓣膜疾病、心肌或大血管问题如肺栓塞或主动脉剥离。神经系统调控引起的昏厥发生于当血管扩张时，而心率却不适当地降低。其通常为诱发事件导致，例如看到鲜血、疼痛、强烈感觉，或特定活动如排尿、呕吐、咳嗽等。此外，当颈动脉窦受压迫时也可能导致神经系统调控引起的昏厥。姿势性低血压为站起时血压降低，通常与药物相关，但也可能为脱水、失血、感染所引发。

Syncope, also known as fainting, passing out and swooning, is defined as a short loss of consciousness and muscle strength, characterized by a fast onset, short duration, and spontaneous recovery. It is due to a decrease in blood flow to the entire brain usually from low blood pressure. Some causes have prodromal symptoms before the loss of consciousness occurs. These symptoms may include lightheadedness, sweating, pale skin, blurred vision, nausea, vomiting, and feeling warm, among others. Syncope may also be associated with a short episode of muscle twitching. If a person does not completely lose consciousness and muscle strength it is referred to as presyncope. It is recommended that presyncope be treated the same as syncope.