The noun for diastolic is diastole. (The final letter in "diastole" is pronounced as a long "e" as in "lee.")
The diastolic pressure is specifically the minimum arterial pressure during relaxation and dilatation of the ventricles of the heart when the ventricles fill with blood.
In a blood pressure reading, the diastolic pressure is typically the second number recorded. For example, with a blood pressure of 120/80 ("120 over 80"), the diastolic pressure is 80. By "80" is meant 80 mm Hg (millimeters of mercury).
"Diastolic" came from the Greek diastole meaning "a drawing apart." The term has been in use since the 16th century to denote the period of relaxation of the heart muscle.