diastole

n. time between two contractions of the heart, time when the heart expands and fills with blood

Diastole is the period of time when the heart relaxes after contraction. Ventricular diastole is the period during which the ventricles are relaxing, while  atrial diastole is the period during which the atria are relaxing.
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Diastole /di·as·to·le/ the dilatation, or the period of dilatation, of the heart, especially of the ventricles.diastol´ic di·as·to·le n. [more]Diastole - Community and Resources

Noun
1. the widening of the chambers of the heart between two contractions when the chambers fill with blood
(hypernym) pulse, pulsation, heartbeat, beat

n. diastole, time between two contractions of the heart