Preposition
Between.
Source: Webster's dictionaryDrake, he's in his hammock till the great Armadas come, (Captain, art tha sleepin' there below?) Slung atween the roundshot, list'ning for the drum, And dreamin' arl the time o' Plymouth Hoe. Henry Newbolt
While clombe above the Eastern bar The horned Moon, with one bright Star Almost atween the tips. Samuel Taylor Coleridge