An aperture is an opening, the term being used to describe the light-collecting opening of a telescope, and also used analogously for radio telescopes and telescopes for other parts of the electromagnetic radiation spectrum. The size is generally listed as a diameter, e.g., in meters, which presumes they are circular, which is generally but not always the case. The width of the aperture affects the instruments angular resolution, due to diffraction, while its area corresponds to the telescope's light-collecting ability, i.e., how dim an object it can discern and how long it takes doing so. If a telescope's aperture is not circular, an equivalent aperture may be cited, generally that of a circle with the same area, but for an interferometer-array of telescopes, its longest baseline may be cited as an equivalent aperture, to suggest the width of a single telescope's aperture that would achieve the same angular resolution as the interferometer.
The current technique of apodization consists of deliberately reducing and/or reshaping the aperture (changing a width along some direction) in order to change the resulting diffraction patterns and tease out detail that diffraction would normally hide.