Blood pressure is the pressure of the blood against the walls of the arteries. When that pressure is consistently above the normal range, it is considered hypertension, also known high or raised blood pressure. Optimal blood pressure is less than 120 mm Hg when the heart beats (systolic) and 80 mm Hg when the heart relaxes (diastolic). When systolic blood pressure is equal to or above 140 mm Hg and/or a diastolic blood pressure equal to or above 90 mm Hg the blood pressure is considered to be raised or high.