ETYM Latin apertura, from aperire. Related to Aperient.
1. An opening in something.
2. An opening; usually small.
3. Controls amount of light admitted.
In photography, an opening in the camera that allows light to pass through the lens to strike the film. Controlled by shutter speed and the iris diaphragm, it can be set mechanically or electronically at various diameters.

1. An open or empty space in or between things; SYN. gap.
2. A vacant or unobstructed space
3. A ceremony accompanying the start of some enterprise.
4. Opportunity especially for employment or promotion
5. The act of opening something
6. The first performance (as of a theatrical production); SYN. opening night, curtain raising.
7. The initial part of the introduction
8. Becoming open or being made open