Hypotension
Hypotension refers to an abnormally low blood pressure. This is best understood as a physiologic state, rather than a disease. For most individuals, a healthy blood pressure lies between 90/50 mmHg to 135/90 mmHg. A small drop in blood pressure, even as little as 20 mmHg, can result in short term hypotension. Decreased blood pressure is often associated with shock, though not necessarily indicative of it. Hypotension is not to be confused with hypertension, which is high blood pressure, the opposite of hypotension.
Reduced blood volume, called hypovolemia, is the most common mechanism producing hypotension in people. This can be due to a traumatic injury, sepsis, acidosis, or medications. Major symptoms of hypotension are lightheadedness or dizziness. If the blood pressure is sufficiently low, fainting and often seizures will occur.
Reference: http://en.wikipedia.org/wiki/Low_blood_pressure