Diastole

Diastole (pronounced di-as´to-le, rhymes with "potentially") 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. You can always remember which is which (diastole and systole) because the Greek word diastole meaning "dilation" shares the same prefix, "Di".

Inside the heart
During ventricular diastole, the pressure in the (left and right) ventricles drops from the peak that it reaches in systole. When the pressure in the left ventricle drops to below the pressure in the left atrium, the mitral valve (atrioventricular bicuspid valve) opens, causing accumulated blood from the atrium to flow into the ventricle.

Inside the arteries
The adjective "diastolic" is used to refer to the relaxation of the heart between muscle contractions. It is used to describe portions of the cardiac cycle related to contraction. More typically it is used as one component of measurement of blood pressure. "Diastolic pressure" refers to the lowest pressure within the arterial blood stream occurring during each heart beat. The other component of blood pressure is systolic pressure, which refers to the highest arterial pressure during each heart beat. When stating blood pressure, systole and then diastole is mentioned; for example: 120/80.