Arterial hypertension is a condition characterized by abnormally high systolic and/or diastolic blood pressure levels. Systolic and diastolic blood pressure levels are usually estimated indirectly by use of an inflatable rubber bladder to compress the artery in the upper arm. The pressure exerted on the artery by the bladder is registered by a device called a manometer. The level of the blood pressure is a measure of the force exerted against the walls of the artery during each heart beat or pulse. The peak of the pressure wave occurs when the heart beats or contracts (systole) and is called the systolic pressure. The valley of the pressure wave occurs when the heart relaxes (diastole) and is termed the diastolic pressure. Blood pressure is recorded as systolic over diastolic.

Although longitudinal research studies such as the one in Framingham, Massachusetts, and the life insurance industry have noted that even slight elevations of blood pressure are associated with increased risk of premature death, the World Health Organization has recommended that the following blood pressure levels be used to classify adults (blood pressure levels defining juvenile hypertension are under review):

Hypertensive: Greater than or equal to 160 millimeters of mercury (mmHg) systolic and/or greater than or equal to 95 mmHg diastolic.

Normotensive: Less than or equal to 140 mmHg systolic and less than or equal to 90 mmHg diastolic.

Borderline Hypertensive: Between hypertensive and normotensive. The condition is also divided etiologically into two types: secondary and primary or essential hypertension. Secondary hypertension, resulting from some known cause (including diseases of the kidney or the adrenal glands), represents less than 10 percent of all the cases of the disease.