Blackout Syncope The medical term for this condition is loss of consciousness.  This is actually a symptom and not a disease.  In fact, many diseases may cause "blackout."  Some are benign, such as a vasovagal reaction -- a formal description of what happens to some when they see blood or a hypodermic needle and pass out. Other times, it may an indication of a serious condition, such as Stroke, Subarachnoid Hemorrhage (ruptured Brain Aneurysm), or heart attack.  Go to these specific diseases to learn more.