aperture

Short definition from Wiktionary

Noun

1. 1
   A small or narrow opening, gap, slit, or hole.
2. 2
   A hole which restricts the diameter of the lightpath through one plane in an optical system.
3. 3
   A hole which restricts the diameter of the lightpath through one plane in an optical system: The diameter of such a hole which restricts the width of the lightpath through the whole system. For a telescope, this is the diameter of the objective lens.
4. 4
   The (typically) large-diameter antenna used for receiving and transmitting radio frequency energy containing the data used in communication satellites, especially in the geostationary belt. For a comsat, this is typically a large reflective dish antenna; sometimes called an array.
5. 5
   The maximum angle between the two generatrices.