Blood pressure is the
force of blood against the walls of the
artery. It is usually measured as x over y, where x is the
systolic blood pressure ,
i.e. the maximum pressure exerted when the
heart contracts, and y is the
diastolic pressure which represents the pressure in your
arteries when the heart is at rest.
Blood pressure is generally measured with a sphygmomanometer (also called aneroid monitor, haemomanometer, or hemadynamometer). This is a cuff that temporarily closes the blood flow to the brachial artery. Next, the pressure on the cuff is slowly released. At some point, the blood will start flowing again in a pulse-wise manner, as can be monitored with a stethoscope. This point determines the systolic pressure. The cuff-pressure is further released until the blood flows without interruptions. This point determins the diastolic pressure.
Sustained high blood pressure is called hypertension.