What is Blood Pressure?
Blood pressure is a measure of the force of blood flowing against the walls of the arteries. Arterial blood pressure is constantly changing during the course of the heart’s cycle
The highest pressure in the cycle is called the systolic Blood pressure: the lowest is the Diastolic Blood Pressure.
Both pressure readings, the Systolic and diastolic, are necessary to enable a doctor to evaluate the status of a patient’s blood pressure.

Why is it a Good Thing to Measure Blood Pressure at Home?
Having your blood pressure measured by a doctor can cause anxiety which itself a cause of high blood pressure. As a variety of conditions affect blood pressure, a single measurement may not be sufficient for an accurate diagnosis.
Many factors such as physical activity, anxiety, or the time of day, can influence your blood pressure. Thus it is best to try and measure your blood pressure at the same time each day, to get an accurate indication of any changes in blood pressure. Blood pressure is typically low in the morning and increases form afternoon to evening. It is lower in the summer and higher in the winter.
Blood pressure is measured in millimeters of mercury (mmHg) and measurement are written with the systolic before the diastolic e.g. A blood pressure written as 135/85, is referred to as 135 over 85 mmHg


Classification of blood Pressure by the World Health Organization
The World Health Organization (WHO) and the International Society of Hypertension (ISH) developed the Blood Pressure Classification shown in this figure



This classification is based on the blood pressure values measured on people In a sitting position in outpatient departments of hospitals.
Note:
There is no universally accepted definition hypotension. However, those having the systolic pressure below 100 mmHg are assumed as hypotensive.

From OMRON health care manual