headspace

Etymology

head + space

noun

1. The space between the top of the contents of a container (such as a jar) and its seal (such as a lid).
   Control of headspace is important for economic and technical reasons. Underfilling will contravene Weights and Measures legislation and may cause rusting in the headspace […] 1997, S. D. Holdsworth, “Food Preservation Processes”, in M. D. Ranken, R. C. Kill, C. Baker, editors, Food Industries Manual, 24th edition, Blackie Academic and Professional, page 517
2. (informal) One's mental state.
   It's like, I need to clear my headspace but I can't do that with him around because he's just ego, you know? He dumps on me. So that's it. 1996, Andrew Cowan, Common Ground
3. (informal) Unscheduled time for reflection and thinking.
   As a business owner, a truly effective business owner that is building a profitable business machine, you need headspace, thinking time, planning time, brainstorming time, education time, decision making time, questioning time and more thinking time. 2011, Eric J. Gregory, Would You Like Profits
4. (firearms) The gap between the face of the bolt and the stopping surface for the cartridge.
5. (chemistry) The gas phase in contact and in equilibrium with a nonvolatile sample.

verb

1. (firearms) To occupy a headspace.