noun

1. An opening, gap, or hole, usually small and narrow

   “an aperture in a wall”

2. Something which restricts the diameter of the light path through one plane in an optical system.

3. The diameter of the aperture (in the sense above) which restricts the width of the light path through the whole system. For a telescope, this is the diameter of the objective lens.

   “This telescope has a 100cm aperture.”