1815, "an interval", also "a person who intervenes". Noun use of prepositional phrase, from in (adv.) + between (prep., adv.).
Between: Old English betweonum, Mercian betwinum
1. Done together or reciprocally
2. Shared in confidence
3. In transit from (one to the other, or connecting places)
4. In the position or interval that separates (two things), or intermediate in quantity or degree.
Photo taken at Casa Árabe (Madrid) // Museum(scape)s series