The quality or state of being between; the condition of existing in the space, time, or position between two things.
From 'between' (Old English 'betweonum,' literally 'by two') plus the suffix '-ity' (from Latin '-itas') that creates abstract nouns. This is a philosophical or technical term for the abstract concept of being in a middle state.
Philosophers and scientists love abstract nouns like 'betweenity'—they let them discuss ideas like 'in-between-ness' as actual things to study, which is why quantum mechanics talks about particles existing in 'superposition,' a kind of ultimate betweenity.
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