The diaphragm is the part of a camera lens which is made up of a series of metal blades which create a hole.
The opening regulates the amount of light which reaches the image sensor.
Commonly it is also known as an iris diaphragm due to its similarity to the human iris.
The blades are arranged in a circular shape and they move to open and close the size of the opening.
The hole that is created is known as the aperture and this affects things like the amount of light let through, as well as the depth of field in your images.
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See the video below for a visual representation of how the diaphragm operates to affect the aperture.