In a blood pressure reading, the diastolic number is the force of blood against your artery walls while your heart is at rest (in between heartbeats). The systolic number is the pressure on artery walls while the heart is beating. Your blood pressure reading is normally expressed as the systolic number first and the diastolic number second, 120/80 mmHg, for example.
A reading of 120/80 mmHg to 139/89 mmHg is classified as prehypertension, the point at which blood pressure is becoming too high and beginning to damage arteries. High blood pressure, or hypertension, is any reading at or above 140/90 mmHg.
Ultimately, both systolic and diastolic blood pressure matter. The National Heart, Blood and Lung Institute (NHBLI) of the National Institutes of Health (NIH) defines a patient as having high blood pressure if either systolic or diastolic blood pressure is high.