The technical definition of pressure is force per unit area.  Thus pressure can be measured in pounds per square inch (English units), or in newtons per square meter (metric units).  Pounds per square inch is normally abbreviated as psi, and one newton per square meter is formally called a pascal, or Pa for short.  However, since atmospheric pressure at sea level on the Earth is about 14.7 lbs/in2, or 1.01 x 105 Pa, it is also fairly common to measure pressures in terms of atmospheres, or atm.  That is, 2 atm = twice the Earth's atmospheric pressure, etc.  Last and probably least, it is traditional in meteorology to measure pressure in terms of millimeters of mercury (or worse, inches of mercury).  This refers to how much pressure is needed to support a column of mercury metal in an inverted tube, and dates all the way back to the time of Galileo.  760 mm of mercury, or 29.92 inches of mercury, is equal to one atmosphere.