i·ris / ˈīris/ • n. 1. a flat, colored, ring-shaped membrane behind the cornea of the eye, with an adjustable circular opening (pupil) in the center. ∎  (also iris diaphragm) an adjustable diaphragm of thin overlapping plates for regulating the size of a central hole, esp. for the admission of light to a lens. 2. a plant with sword-shaped leaves and showy flowers, typically purple, yellow, or white. Native to both Eurasia and North America, it is widely cultivated as an ornamental. • Genus Iris, family Iridaceae (the iris family): many species and numerous hybrids, including the crested dwarf iris (I. cristata) and the sweet iris (I. pallida). The iris family also includes the gladioli, crocuses, and freesias. 3. a rainbow or a rainbowlike appearance.• v. [intr.] (of an aperture, typically that of a lens) open or close in the manner of an iris or iris diaphragm.

Iris (family Iridaceae) A genus of plants that have rhizomes or bulbs, leafy shoots usually flattened in one plane, and showy, characteristic flowers in which the 3 outer perianth segments (the ‘falls’) usually arch down and are larger than the 3 inner perianth segments (the ‘standards’) which are often more or less erect and stalked. The 3 style-branches are broad and petal-like, and forked at the apex. There are some 300 species, throughout the northern temperate zone, many being cultivated (as are many hybrids) for their beautiful flowers.

iris The pigmented ring of muscular tissue, lying between the cornea and the lens, in the eyes of vertebrates and some cephalopod molluscs. It has a central hole (the pupil) through which light enters the eye and it contains both circular and radial muscles. Reflex contraction of the former occurs in bright light to reduce the diameter of the pupil (see pupillary reflex); contraction of the radial muscles in dim light increases the pupil diameter and therefore the amount of light entering the eye. Colour is determined by the amount of the pigment melanin in the iris. Blue eyes result from relatively little melanin; grey and brown eyes from increasingly larger amounts.

iris Genus of about 300 species of flowering plants widely distributed, mostly in temperate areas. They are monocotyledons and have bulbs or rhizomes. Height: up to 90cm (3ft). Family Iridaceae. See also crocus; gladiolus

iris (I-ris) n. the part of the eye that regulates the amount of light that enters. It forms a coloured muscular diaphragm across the front of the lens; light enters through a central opening (the pupil). Contraction of different sets of muscles of the iris causes the pupil to dilate in dim light and to contract in bright light. i. bombé an abnormal condition of the eye in which the iris bulges forward towards the cornea.