Syncope (pronounced SIN-ko-pee) is the modern medical term for fainting, i.e., the sudden loss of consciousness and posture due to lack of blood to the brain. This is most likely due to sudden emotional stress, postural hypotension, or loss of blood.
This serves the physiological purpose of shutting down non-essential functions and allowing the body to focus on redirecting blood supply to the brain. As it happens, falling down makes it easier for the heart to pump blood to the brain, as it is no longer fighting gravity. Moreover, the victim will usually start breathing faster and the heart rate will increase, although blood pressure may still be low in the extremities, as most of the increased blood flow will be directed to the brain.
There are specific terms for different causes of syncopes:
A neurocardiogenic syncope or vasovagal syncope is caused when the vagus nerve overreacts to an emotional trigger, and heart rate and blood pressure drop suddenly. Common triggers include the sight of blood, a sudden fright, or receiving bad news.
An occupational syncope follows from a purely physical cause, rather than emotional. This may sometimes be a case of a healthy person overexerting themselves, but usually results from an unhealthy person overexerting themselves through coughing, defecating, or sneezing.
A cardiovascular syncope results when damage to the heart prevents the heart from pumping enough blood to the brain. This would include a heart attack, among other conditions.
Orthostatic hypotension results from standing up too quickly, and the heart not increasing blood pressure quickly enough to maintain blood supply to the head. This is more common when a person is dehydrated, which may be secondary to a medical condition such as diabetes. Orthostatic hypotension is often blamed for fainting due to overconsumption of alcohol, although alcohol causes a wide range of effects that both make hypotension more likely and also mimics a vasovagal syncope. In most cases orthostatic hypotension results in dizziness (AKA a presyncope), but may cause fainting in some cases.
A cerebrovascular syncope occurs when there is a problem with the blood vessels bringing blood to the brain, most often as a result of cerebrovascular disease (e.g., stroke). This would also include any case where someone passes out due to compression of the carotid artery, as is used in some martial arts.
In addition, there are various random conditions that can cause syncope, and in many people, especially the elderly, multiple factors contribute to a syncope. There are other causes of losing consciousness that are not considered syncope, including seizure, concussion, low blood oxygen, low blood sugar, and drug intoxication. In any case, where the cause of loss of conscious is not immediately apparent and benign, you should see a doctor immediately.