irises Definition

iris
noun

irises, irides
1. technical
) a perennial plant that arises from a rhizome or a corm, and has flattened sword-shaped leaves and large brilliantly coloured flowers, consisting of equal numbers of upright and hanging petals.
2. anatomy.
An adjustable pigmented ring of muscle lying in front of the lens of the eye, surrounding the pupil, and which, through contraction or relaxation of its muscle fibres, increases or decreases the size of the pupil, thereby controlling the amount of light entering the eye.
3. A device consisting of a series of thin overlapping crescent-shaped plates surrounding a central circular aperture, used to control the amount of light entering an optical instrument.

Form: iris diaphragm (in full)

Etymology: 14c: Greek, meaning ‘rainbow'.