- An opening, such as a hole, gap, slit, vent, port, mouth, outlet, or perforation, through which electrons, light, waves, or any form of radiant energy may pass. Also called aperture (1).
- In a waveguide, resonant cavity, loudspeaker, or other similar enclosure, the opening through which energy is transmitted.
- noun an opening in the body, e.g. the mouth or anus