1. Symbol I A lustrous, purple-black, corrosive, poisonous halogen occurring as a diatomic molecule, I₂, that easily sublimes to give a purple gas and is a trace element essential for proper thyroid function. Radioactive isotopes, especially I-131, are used as medical tracers and in thyroid disease diagnosis and therapy. Iodine compounds are used as germicides, antiseptics, and dyes. Atomic number 53; atomic weight 126.9045; melting point 113.7°C; boiling point 184.4°C; density of gas 11.27 grams per liter; specific gravity (solid, at 20°C) 4.93; valence 1, 3, 5, 7. See Periodic Table.

2. An antiseptic preparation containing iodine in solution, used to treat wounds.