What is precipitable water vapor in millimeter-wave radioastronomy and how is it measured?
As defined quite clearly, with equations, here

The total atmospheric water vapor contained in a vertical column of
unit cross-sectional area extending between any two specified levels,
commonly expressed in terms of the height to which that water
substance would stand if completely condensed and collected in a
vessel of the same unit cross section.

edit
There's, for example, an ALMA paper which discusses measurements and comparison with meteorological stations.