WordNet sense 1 (an open or empty space in or between things):

gap

WordNet sense 2 (the first performance (as of a theatrical production)):

curtain raising, opening night

WordNet sense 3 (a possible alternative):

possibility, possible action

WordNet sense 4 (an aperture or hole that opens into a bodily cavity):

orifice, porta

WordNet sense 5 (an entrance equipped with a hatch; especially a passageway between decks of a ship):

hatchway, scuttle

WordNet sense 6 (serving to set in motion):

initiative

WordNet sense 7 (serving to begin):

beginning

WordNet sense 8 (a recognized sequence of moves at the beginning of a game of chess):

chess opening

WordNet sense 9 (the first of a series of actions):

first step, opening move