iodine stain

n.
  1. A stain used to detect a variety of polysaccharides, such as cellulose, chitin, and glycogen, in which potassium iodide reacts with starch to produce a blue color.
The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.