From Glossary of Meteorology
(Also called barometric tendency.) The character and amount of atmospheric pressure change during a specified period of time, often a three-hour period preceding an observation.
Pressure tendency is composed of two parts, the pressure change and the pressure characteristic. The pressure change is the net difference between pressure readings at the beginning and ending of a specified interval of time. The pressure characteristic is an indication of how the pressure has been changing during that specified period of time, for example, decreasing then increasing, or increasing and then increasing more rapidly.