1. A dye or other coloring material that is used in microscopy to make structures visible.
2. A soiled or discolored appearance; SYN. discoloration, discolouration.
3. In chemistry, a colored compound that will bind to other substances. Stains are used extensively in microbiology to color microorganisms and in histochemistry to detect the presence and whereabouts in plant and animal tissue of substances such as fats, cellulose, and proteins.

1. To mark with dye, as of furniture
2. To produce or leave stains