What do the two numbers in a blood pressure reading mean?
Blood pressure is the pressure exerted by the blood onto the walls of the arteries. The amount of pressure that exists varies as the heart beats. It is at its highest when the heart pumps, and at its lowest when the heart relaxes. Those two states are referred to, respectively, as systolic pressure and diastolic pressure. By convention, the former is written over the latter; thus, a blood pressure of 120/80 indicates a systolic pressure of 120 mm Hg (millimeters of mercury, the units in which blood pressure is measured), and a diastolic pressure of 80 mm Hg.
To be clear, the two numbers aren't used as a ratio; one is considered to have high blood pressure if either number is above the normal range, even if the other is within normal bounds.
—The Fact Monster