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Merriam-Webster Online Dictionary
diastole (noun)
a rhythmically recurrent expansion , especially the dilatation of the cavities of the heart during which they fill with blood
Diastole (Wikipedia)
For the Greek punctuation mark sometimes known as the diastole, see hypodiastole.
Heart during ventricular diastole.

Diastole /dˈæstəl/ is the part of the cardiac cycle when the heart refills with blood following systole (contraction). Ventricular diastole is the period during which the ventricles are filling and relaxing, while atrial diastole is the period during which the atria are relaxing. The term diastole originates from the Greek word διαστολη, meaning dilation.