1. To discolor, soil, or spot.
2. To bring into disrepute; taint or tarnish.
3. To color (glass, for example) with a coat of penetrating liquid dye or tint.
4. To treat (specimens for the microscope) with a reagent or dye that makes visible certain structures without affecting others.

1. A discolored or soiled spot or smudge.
2. A blemish on one's moral character or reputation.
3. A liquid substance applied especially to wood that penetrates the surface and imparts a rich color.
4. A reagent or dye used for staining microscopic specimens.

Synonyms: These nouns denote a mark of discredit or disgrace, as on one's good name: a stain on his honor; the blot of treason; the brand of cowardice; the stigma of ignominious defeat; the taint of vice.