A small or narrow opening, gap, slit, or hole.
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.
A hole which restricts the diameter of the lightpath through one plane in an optical system.
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.
The maximum angle between the two generatrices.
In an organ flue pipe, the opening between the lower lip and the languet.
A pipe or duct that carries gaseous combustion products away from the point of combustion (such as a furnace).
An enclosed passageway in which to direct air or other gaseous current along.