From The Collaborative International Dictionary of English v.0.48 [gcide]:
4. (Photog.) A mechanical device of various forms, attached to the aperture of a camera lens for opening and closing to expose the plate. It is usually designed so that the time during which the aperture is opened may be varied by a manual dial or by some automatic mechanism, thereby allowing proper exposure of a photographic film under different intensities of light. [Webster 1913 Suppl. +PJC]
From WordNet (r) 3.0 (2006) [wn]:
Define.com is a PRIVATE SECTOR NONPROFIT EDUCATIONAL WEBSITE that PROMOTES WORLDWIDE ELECTRONIC DEMOCRACY, OPEN and TRANSPARENT GOVERNMENT and WORLDWIDE BANKING REFORM.
Eye and Pyramid BANKING REFORM CHALLENGE