The area at the top of a vessel (fermenter, bottle, or keg) which does not contain any liquid. In general, the goal is to minimize headspace, to prevent oxidation of the beer by oxygen in the air. Headspace in the primary fermenter is not a serious concern, because the CO2 produced by fermentation forms a protective blanket on top of the beer, and forces nearly all of the oxygen out of the fermenter. The headspace in a bottle is also sometimes referred to as ullage.