Aperture

Aperture is the adjustable opening inside a lens that controls how much light passes through to the sensor, expressed as an f-number (f/2.8, f/8, f/16, and so on) — the ratio of the lens's focal length to the diameter of that opening. A lower f-number means a physically wider opening and more light passing through; a higher f-number means a narrower opening and less light.

Aperture has two separate, equally important effects: it's one of the three exposure-triangle variables controlling how bright or dark an image comes out, and it directly controls depth of field — wider apertures (lower f-numbers) produce shallower depth of field, and narrower apertures (higher f-numbers) produce more of the scene in acceptable focus, right up until diffraction starts working against overall sharpness at very small apertures.