definition
An opening, gap, or hole, usually small and narrow
example
an aperture in a wall
definition
Something which restricts the diameter of the light path through one plane in an optical system.
definition
The diameter of the aperture (in the sense above) which restricts the width of the light path through the whole system. For a telescope, this is the diameter of the objective lens.
example
This telescope has a 100cm aperture.
definition
(communications) 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.
definition
(of a right circular cone) The maximum angle between the two generatrices.
example
If the generatrix makes an angle θ to the axis, then the aperture is 2θ.