What is the meaning of Headspace?

The space between the top of the contents of a container (such as a jar) and its seal (such as a lid).

One's capacity for mental activity modeled as a physical space with limits; one's mental state or state of mind.

Time and opportunity for reflection and thinking.

The gap between the face of the bolt and the stopping surface for the cartridge.

The gas phase in contact and in equilibrium with a nonvolatile liquid in a sample.

To occupy a headspace.

Source: wiktionary.org