The Collaborative International Dictionary of English v.0.48:

Iris diaphragm \I"ris di"a*phragm\
   An adjustable diaphragm, reminiscent of the iris of the eye
   in its action, by which the diameter of an approximately
   circular opening may be controlled, as for regulating the
   aperture of a lens; it consists of a number of movable thin
   curved plates fastened at regular intervals around the inside
   of a ring, the positions of which are simultaneously adjusted
   by a single knob on the outside of the lens. It is used in
   cameras and microscopes.
   [Webster 1913 Suppl. +PJC]

WordNet (r) 3.0 (2006):

iris diaphragm
    n 1: diaphragm consisting of thin overlapping plates that can be
         adjusted to change the diameter of a central opening [syn:
         iris, iris diaphragm]