When your heart beats, it squeezes and relaxes; diastole is when it relaxes and fills with blood.
Since the 16th century, doctors have used the Greek word diastole, or "dilation," for the stage in the cardiac cycle when the heart muscle relaxes and its ventricles dilate, or widen. The contraction that follows is called systole. Every beat of your heart is a repetition of these two phases, with a drop in blood pressure during diastole as its chambers are refilled.