Aperture
Part of speech: noun
Definition: An opening; an open space; a gap, cleft, or chasm; a passage perforated; a hole; as, an aperture in a wall. Something which restricts the diameter of the light path through one plane in an optical system. The diameter of the aperture (in the sense above) which restricts the width of the light path through the whole system. For a telescope, this is the diameter of the objective lens. e.g. a telescope may have a 100 cm aperture.
Iris
Part of speech: noun
Definition: A plant of the genus Iris, common in the northern hemisphere, and generally having attractive blooms.The contractile membrane perforated by the pupil, which adjusts to control the amount of light reaching the retina, and which forms the colored portion of the eye.A diaphragm used to regulate the size of a hole, especially as a way of controlling the amount of light reaching a lens.A rainbow, or other colourful refraction of light.