Liquid water, frost, or visible moisture forming on a surface inside a nominally sealed glazing cavity rather than on an exposed room-side or weather-side surface.
Michiel1972 · Image source · CC BY-SA 3.0 · Image source · License
Meaning and boundaries
Condensation on an exposed surface can be normal under dew-point conditions and should not be mislabeled as internal IGU failure.
Why it matters in glass restoration
Cavity moisture is inaccessible to surface restoration and commonly requires glazing-system diagnosis or unit replacement.