In any optical system, an opening allowing the passage of light, electrons, or other forms of radiation. In photography, the aperture is the hole through which light passes into a camera, the size of which is controlled by the diaphragm. (See Photography.) An aperture, in this sense, is also known as an iris.
In a cathode-ray tube, the size of the aperture determines the size and shape of the electron beam.
In screen printing, the term aperture refers to one specific opening in the mesh screen through which ink passes, also known as a mesh aperture. (See Screen Printing.)