In a lens the aperture is the orifice, usually an adjustable iris, which limits the amount of light passing through a lens. In a projector the aperture is a mask opening that defines the area of the frame projected.
A useful way of controlling the amount of light passing through a lens is to use an iris diaphragm. This usually consists of a number of metal leaves that are overlapped to form a symmetrical opening of approximately circular shape.
In a cinema motion picture projector, the lens is normally kept wide open and the aperture is in the form of a metal ‘plate’ with a cut-out at the same aspect ratio as the format.