Lose consciousness is a term used to describe a state of unconsciousness, usually caused by a lack of oxygen or a sudden drop in blood pressure. Fainting, swooning, and passing out are all terms used to describe this state. Fainting is a sudden, brief loss of consciousness due to a decrease in blood flow to the brain. Swooning is a similar phenomenon, but it is usually caused by an emotional reaction such as fear or excitement. Passing out is a more severe form of losing consciousness, usually caused by a medical condition or injury.