Douglas Harper's Etymology Dictionary
betweenness
1881, from between + -ness.
Wiktionary
betweenness
n. The state or quality of being between.
Wikipedia
Betweenness
Betweenness is an algorithmic problem in order theory about ordering a collection of items subject to constraints that some items must be placed between others. It has applications in bioinformatics, but was shown to be NP-complete by .