Webster's English Dictionary

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aperture

ap.er.ture \'ap-*(r)-.chu.(e)r, -ch*r, -.t(y)u.(*)r\ n [ME, fr. L apertura, 
   fr. apertus, pp. of aperire to open -]more at WEIR 1: an opening or open 
   space : HOLE  2a: the opening in a photographic lens that admits the light  
   2b: the diameter of the stop in an optical system that determines the diameter
   of the bundle of rays traversing the instrumentgh. APERTURE applies to 
   any opening in an otherwise solid wall or surface; INTERSTICE applies to an 
   unfilled gap or interval in a mass or a fabric; ORIFICE applies to an 
   opening suggesting a mouth or a vent SYN syn APERTURE, INTERSTICE, ORIFICE 
   mean a passage throu