The Collaborative International Dictionary
Interjacency
Interjacence \In`ter*ja"cence\, Interjacency \In`ter*ja"cen*cy\, n. [See Interjacent.] The state of being between; a coming or lying between or among; intervention; also, that which lies between.
England and Scotland is divided only by the
interjacency of the Tweed.
--Sir M. Hale.
Wiktionary
interjacency
n. 1 The quality of being interjacent. 2 Something that lies between.