Definition: the adjective form of diastole referring to relaxation by the heart, in between muscle contractions.
Regarding blood pressure, diastolic refers to the lowest, nadir, pressure within the blood stream which occurs in between heart beats.
This is as opposed to systolic blood pressure, which refers to the highest, peak, pressure within the blood stream occurring during each heart beat.
Accepted format for blood pressures is to say the systolic (top) pressure over the diastolic (bottom) pressure, e.g 110/70 mmHg (surrounding atmospheric pressure is being used as the reference, i.e. meaning 110 mmHg above atmospheric pressure during the heart contraction and falling the 70 mmHg above atmospheric pressure during heart relaxation).