noun

• A small or narrow opening, gap, slit, or hole.
• (optics) A hole which restricts the diameter of the lightpath through one plane in an optical system.
• (astronomy, photography) 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.
• (space flight, communication) 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.
• (mathematics, rare, of a right circular cone) The maximum angle between the two generatrices.