iodine

n. nonmetallic element of the halogen group used in medicine and photography (Chemistry); powdered iodine mixed with ethyl alcohol to form an antiseptic solution

Iodine (, or ; from "violet"), is a chemical element that has the symbol I and atomic number 53. Chemically, iodine is the least reactive of the halogens, and the most electropositive halogen after astatine. Iodine is primarily used in medicine, photography and dyes. It is required in trace amounts by most living organisms.
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(n.)
A nonmetallic element, of the halogen group, occurring always in combination, as in the iodides. When isolated it is in the form of dark gray metallic scales, resembling plumbago, soft but brittle, and emitting a chlorinelike odor. Symbol I. Atomic weight 126.5. If heated, iodine volatilizes in beautiful violet vapors.
  

Essential element in the diet. The thyroid gland uses iodine to make thyroid hormones. The two most important thyroid hormones are thyroxine (T4) and triiodothyronine (T3). Thyroxine (T4) has four iodine molecules attached to its structure, while triiodothyronine (T3) has three iodine molecules attached. Iodine is found in seafood, bread, salt, and seaweed.

An element that is necessary for the body to make thyroid hormone. It is found in shellfish and iodized salt.