1.NounA small or narrow opening, gap, slit, or hole.
"an aperture in a wall"
2.NounA 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.
"This telescope has a 100 cm aperture."
3.NounThe 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.
"This telescope has a 100 cm aperture."
4.NounThe (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.NounThe maximum angle between the two generatrices.
"If the generatrix makes an angle θ to the axis, then the aperture is 2θ."