Blood pressure is the force by which the blood pushes against the artery walls. High blood pressure, or hypertension, is when this force becomes too high.
Blood pressure is measured in millimeters of mercury, and there are two numbers used to assess blood pressure: systolic and diastolic.
Systolic blood pressure (the top number) is the blood pressure when the heart beats, while diastolic blood pressure (the bottom number) is the blood pressure between heartbeats, when the heart is at rest.
Previous guidelines — established in 2003 — defined hypertension as a blood pressure of 140/90 millimeters of mercury or higher.
Having a blood pressure of between 120/80 millimeters of mercury and 139/89 millimeters of mercury was categorized as prehypertension, whereby blood pressure levels are higher than normal, but are not high enough to be considered hypertensive.