What the definition of systolic and dystolic?

When taking blood pressure there are two readings, eg 120/70. The first reading (120) is the systolic blood pressure and the second reading (70) is the diastolic blood pressure. The systolic blood pressure is always higher than the diastolic.

The systolic reading is when the heart muscle is contracting, pushing blood through the arteries, therefore creating more pressure (hence the higher number) in the arteries.

The diastolic reading is when the heart is relaxed and refilling with blood, ready for the next beat, therefore exerting less pressure on the arteries (hence the lower number)