Diastolic pressure is the pressure of blood against the artery walls between heartbeats, when the heart is relaxed and filling with blood. It is the second or lower number in a blood pressure reading.

For example, if the diastolic pressure is 80 millimeters of mercury (mm Hg) and the systolic pressure is 120 mm Hg, the blood pressure is recorded as 120/80 and read as "120 over 80."