When we talk about blood pressure we are talking about the amount of pressure exerted on your blood vessels when your heart takes a beat (systolic pressure) and when it rests in between beats (diastolic pressure) while you are in a resting state (i.e: sitting quietly).
We measure this using a blood pressure cuff which wraps firmly around the upper part of the arm. It can be measured using a digital or manual sphygmomanometer (say: sfig-mow-man-om-meter), aka: blood pressure gauge. A digital gauge requires simply pushing a button, where as a manual gauge requires a stethoscope to be placed against the brachial artery and the result read according to the korotkoff sounds. This process should be painless, but the pressure can be uncomfortably tight when the cuff is inflated - especially if the blood pressure is high. Technique can manage this if a manual cuff is used and the the size of the cuff is important for accuracy and comfort - it should cover two thirds of the upper arm, and be applied firmly but not tightly to give the most accurate result. Very low blood pressures or very irregular heart rates are usually best taken manually.
The results are measured in mmHg (millimeter of mercury - a manometric measure of pressure). The original gauges were tubes filled with mercury (some practices still use them), but thankfully progress has deemed them no longer necessary. The results show the systolic blood pressure (pressure when the heart beats) over the diastolic pressure (pressure when the heart rests in-between). The ideal blood pressure is expressed as 120/80mmHg: 120 being the systolic and 80 being the diastolic.
High Blood Pressure:
High blood pressure is any systolic reading over 140mmHg. One singular reading is not really indicative of a blood pressure problem as it could be temporarily high due to:
Viral or bacterial infection will raise the systolic pressure
Heightened anxiety or stress
White Coat Syndrome
Coffee or caffeinated drink
To suspect a chronic high blood pressure the BP would be measured at least three times on different days, or preferably, a week apart.