an opening, as a hole, slit, crack, gap, etc.

Also called aperture stop. Optics . an opening, usually circular, that limits the quantity of light that can enter an optical instrument.

noun

breach , break , chasm , chink , cleft , crack , cut , eye , fissure , gap , gash , interstice , opening , orifice , outlet , passage , perforation , pinhole , puncture , rift , rupture , slash , slit , slot , space , vent , mouth , fenestration , hiatus , hole , leak , os , ostiole , pore , window