Fainting, also called syncope, is a sudden, brief / temporary loss of consciousness and posture caused by decreased blood flow to the brain.
Fainting may have a variety of causes (just to name a few):
Low blood sugar
Standing up too fast
Before fainting, you may experience:
Rapid heartbeat or palpitations
If you think you're going to faint:
Lie Down. Allow blood to circulate to the brain. When you feel better move to a sitting position for several minutes with your head lowered forward between your knees. When you stand up again do it slowly.
Keep yourself hydrated. If you are in a hot environment and your body is losing more water due to sweating, make sure to drink enough fluids.
Keep blood circulating.
If you have to stand or sit for a long time, periodically tense your leg muscles or cross your legs to help improve blood return to the heart and brain.
If someone else faints:
Lay the person flat on their back and elevate the person's legs above heart level, but avoid moving the person if you think he or she might have been injured when falling (moving an injured person can do more damage than good).
Loosen any constrictive clothing like belts, collard, or ties to help restore blood flow.
Check the person's airway to be sure it's clear. Check for signs of circulation (breathing, coughing or movement). If absent, begin CPR. Continue CPR until the person responds and begins to breathe.
Someone who has fainted will usually recover quickly. Because it's normal to feel a bit weak after fainting, be sure the person stays lying down. Getting up too quickly may bring on another fainting spell.