You searched for: “aperture”
aperture (s) (noun), apertures (pl)
The clear diameter of the light-gathering surface of a telescope: For a refractor telescope, the aperture is the diameter of the objective lens, and for a reflecting telescope, it is the diameter of the primary mirror.

This entry is located in the following unit: Astronomy and related astronomical terms (page 2)
aperture, apertures
A hole or an opening. (2)
This entry is located in the following unit: Word a Day Revisited Index of Cartoons Illustrating the Meanings of Words (page 12)