1. The period of time when the heart refills with blood after systole (contraction). (wikipedia.org)
2. The phase of the heartbeat when the heart muscle relaxes and allows the chambers to fill with blood. (Google Dictionary)
3. The widening of the chambers of the heart between two contractions when the chambers fill with blood. (wordnetweb.princeton.edu)
4. The time, in between ventricular contractions (systole), at which ventricular filling occurs. (biology-online.org)
5. (Italicized) A genus of air-breathing land snails or semi-slugs, terrestrial pulmonate gastropod mollusks in the family Helicarionidae.
Word origin: From New Latin diastole, itself based on Ancient Greek διαστολή (diastolē, “separation, drawing asunder”).