Diaphragm consisting of thin overlapping plates that can be adjusted to change the diameter of a central opening WordNet 3.0 & Open English WordNet

An adjustable diaphragm 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. Wiktionary

Adjustable diaphragm. Wiktionary (translation)