What is "precipitable water"

n. The amount of water vapour, in a column of air, that is equivalent to the depth of the liquid water that would be obtained if it fell as rain

'''Precipitable water ''' is the depth of water in a column of the atmosphere, if all the water in that column were precipitated as rain. As a depth, the precipitable water is measured in millimeters or inches. Often abbreviated as "TPW" = total precipitable water.