Diastolic is a term used to describe the pressure in the arteries when the heart is at rest between heartbeats. It is the second number in a blood pressure reading and indicates the pressure when the heart is filling with blood.
For example: If someone has a blood pressure reading of 120/80, the diastolic pressure is 80.
Very Simple Definition:
The diastolic number in a blood pressure reading shows how relaxed the heart is between beats. It is the lower number in the blood pressure reading, like 80 in 120/80.
For example: When a doctor checks your blood pressure, they use two numbers and the bottom one tells how relaxed your heart is when it is not beating.