Fainting is a sudden, temporary, loss of consciousness and muscle strength that results in falling down.

Prior to fainting patients usually complain of feeling dizzy, light headed, losing balance, then start to feel their heart racing and may have cold sweating. Things then start to look dimmer and narrower like a tunnel, and then all of a sudden the person blacks out and falls down. Bystanders usually describe the patient as looking pale, or lost his color.

Syncope is the medical term for fainting. Syncope is therefore a symptom, that could be caused by numerous different underlying conditions, e.g., it could result from something as simple as sight of blood to something as serious as a heart valve problem. Syncope occurs when there is a transient cessation of blood flow to the brain.

Almost fainted!
Some people complain of feeling dizzy, un-steady, and getting out of balance, but do not loss consciousness, this is called near fainting or presyncope.

What causes this transient reduction of blood flow to the brain? How is blood flow to the brain maintained, so we do not faint?
Normally the right side of the heart fills with low-oxygenated blood coming back from the body pumps it to the lungs to get oxygen via the pulmonary artery. The blood, which is now well oxygenated returns to the left side of the heart, that pumps it to the brain through the carotid arteries. So as long as the brain is receiving enough amount of well-oxygenated blood we stay Alert and do not faint.