Noun

1. An open or empty space in or between things
2. A ceremony accompanying the start of some enterprise
3. Becoming open or being made open
4. The first performance (as of a theatrical production)
5. The act of opening something
6. Opportunity especially for employment or promotion
7. The initial part of the introduction
8. A possible alternative
9. An aperture or hole that opens into a bodily cavity
10. A vacant or unobstructed space that is man-made
11. An entrance equipped with a hatch; especially a passageway between decks of a ship
12. A recognized sequence of moves at the beginning of a game of chess
13. The first of a series of actions