An Aperture is an opening through which light passes, 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 (EMR) spectrum. The size is generally listed as a length, e.g., meters, which presumes they are circular, which is generally but not always the case. The size of the aperture affects the instruments Angular Resolution due to Diffraction.
The current technique of Apodization consists of deliberately reducing and/or reshaping the aperture in order to change the resulting diffraction patterns and tease out detail that diffraction would normally hide.
Arecibo Observatory (NAIC)
Extremely Large Telescope (ELT)
European Southern Observatory Very Large Telescope (ESO VLT)
Fast Fourier Transform Telescope
Gran Telescopio Canarias (GTC)
Point-Spread Function (PSF)
Square Kilometre Array (SKA)