A diaphragm (or iris or iris diaphragm) is a mechanism in a camera that makes a variable aperture to control the amount of light that passes through the lens and exposes the film or image sensor.
A diaphragm may take many forms, from very simple devices for "point-and-shoot" film cameras consisting of just two notched pieces of metal, to more complex ones used in higher-quality cameras which have many blades arranged in a circle. This arrangement, also called an "iris" after the corresponding structure in the eye, creates a nearly-circular aperture whose size can be varied as needed. There may be as few as 5 blades to as many as 19. In cameras with a small number of diaphragm blades, the shape of the aperture itself (a pentagon or hexagon) can sometimes be seen in pictures taken towards a strong light source, like the sun.
Usually aperture size control is aided by an f-stop scale.