Blood pressure is the pressure exerted by the blood on the walls of the blood vessels. Unless indicated otherwise, blood pressure is understood to mean arterial blood pressure, i.e., the pressure in the large arteries, such as the brachial artery (in the arm). The pressure of the blood in other vessels is lower than the arterial pressure.

The peak pressure in the arteries during the cardiac cycle is the systolic pressure, and the lowest pressure (at the resting phase of the cardiac cycle) is the diastolic pressure. Typical values for the arterial blood pressure of a resting, healthy adult are approximately 120 mmHg systolic and 80 mmHg diastolic (written as 120/80 mmHg), with large individual variations. Blood pressure is not static, but undergoes natural variations from one heartbeat to another or throughout the day (in a circadian rhythm); it also changes in response to stress, nutritional factors, drugs, or disease.