This is a number that expresses the ability of a lens to resolve fine detail in an object being observed fixed to a slide. It is derived by a mathematical formula (n sine u) and is related to the angular aperture of the lens and the index of refraction of the medium found between the lens and the specimen.

An aperture membrane is a section of enzine forming the base of an aperture.

"Aperture" means "opening".

"Aperture" is a synonym for a break or a crack. "Aperture" is used in many modern day phrases such as camera aperture and of course, "Aperture Science" from the game Portal.

The aperture of my camera lens will not open!