Plain English Meaning
A dirty or smudged windscreen that makes it harder to see clearly, particularly at night when small light sources turn into a fuzzy glare that hides things you would otherwise notice.
Full Definition
A film of dust, oil, smudges, scratches, or moisture on the inside or outside of an aircraft windscreen that scatters incoming light and reduces visual clarity, especially at night when oncoming light from stars, ground sources, or other aircraft spreads into glare and washes out faint objects.
Context Anchor
Encountered during night flying, preflight inspection, and cockpit cleaning, especially when checking whether the windscreen is clear enough for good outside visibility.