Stain has several possible meanings:

• A stain can be an unwanted localized discoloration, often in fabrics or textiles.
• Wood stain is a type of penetrative dye used to create a wanted color change in wood.
• Stains are used in biology and medicine to highlight tissues or cells.
  • In vivo staining is used within living organisms
  • In vitro straining is used on tissue or cells that are no longer within a living organism.