Systemic hypotension - or often just - hypotension - is the presence of an abnormally low blood pressure. This may be relative, so that a poorly-controlled hypertensive patient who runs a normal blood pressure at 160/100, may be severely hypotensive at 80/60.
Hypotension is usually defined with reference to the systolic pressure, however the systolic and diastolic pressures usually change hand-in-hand. When blood pressure decreases pathologically, the pulse pressure (difference between the systolic and diastolic pressures) also tends to narrow. Hypotension is usually seen in the context of shock, most commonly hypovolemic shock.