numerical aperture

noun

nu·​mer·​i·​cal aperture n(y)u̇-ˈmer-i-kəl- 

: a number that indicates the resolving power of a microscope objective and is equal to the product of the index of refraction of the medium (as immersion oil) in front of the objective and the sine of half the angle formed by two rays emanating from the object upon which the microscope is focused and entering the objective at either end of a diameter of the objective