An unintended cavity filled with air in a casting product.
A top-facing opening to a cavity in the ground very near an ocean's shore, leading to a marine cave from which wave water or bursts of air are expelled.
The spiracle, on the top of the head, through which cetaceans breathe.
A vertical opening in the top of a computer case that lets hot air (primarily from the CPU heat sink) escape quickly.
A vent for the escape of steam or other gas.
To fill or be filled with air in an unintended cavity.
A vacant position, especially in an array.
The first few measures of a musical composition.
The first performance of a show or play by a particular troupe.
A gap permitting passage through.
The initial period when a show at an art gallery or museum is first opened, especially the first evening.
An unoccupied employment position.
In mathematical morphology, the dilation of the erosion of a set.
An act or instance of making or becoming open.
A time available in a schedule.
An opportunity, as in a competitive activity.
An act or instance of beginning.
The first few moves in a game.
Pertaining to the start or beginning of a series of events.
describing the first period of play, usually up to the fall of the first wicket; describing a batsman who opens the innings or a bowler who opens the attack