The iris is the colored part of the eye located between the cornea and lens. The iris' round, central opening (the pupil) shrinks and widens to control light coming into the eye. This allows you to see well under different lighting conditions.
Inflammation of the iris is called iritis.
Evans M. Anatomy of the uvea. In: Yanoff M, Duker JS, eds. Ophthalmology. 3rd ed. St. Louis, Mo: Mosby Elsevier; 2009:chap 7.1.