Discolored patches or marks that are difficult to remove; blemishes that spoil the appearance of something.
From Old French esteindre meaning to extinguish or dull, from Latin extinguere. The meaning shifted from dulling brightness to marking with unwanted color.
Stain removal is essentially chemistry - different stains require different solvents because of molecular structure. Red wine needs different treatment than grease because they bond to fabric through completely different mechanisms.
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