Hypertension, or high blood pressure, is a common medical condition where the force of the blood against the walls of the arteries is consistently too high. Blood pressure is measured in millimetres of mercury (mmHg) and is expressed with two numbers: systolic pressure over diastolic pressure. The systolic pressure represents the force when the heart contracts, and the diastolic pressure is the force when the heart is at rest between beats.

Unit 5,

Reference: Oxford Concise Medical Dictionary (9th ed) Oxford University Press 2015 ISBN-13: 9780199687817, p1653