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late 14c., ele, "lateral division of a church (usually separated by a row of pillars), from Old French ele "wing (of a bird or an army), side of a ship" (12c., Modern French aile), from Latin ala, related to axilla "wing, upper arm, armpit; wing of an army," from PIE *aks- "axis" (see axis), via a suffixed form *aks-la-. The root meaning in "turning" connects it with axle and axis.
Confused 15c. with unrelated ile "island" (perhaps from notion of a "detached" part of a church), and so it took an -s- when isle did, c.1700; by 1750 it had acquired an a-, on the model of French cognate aile. The word also was confused with alley, which gave it the sense of "passage between rows of pews or seats" (1731), which was thence extended to railway cars, theaters, etc.
portion of a church or basilica that parallels or encircles the major sections of the structure, such as the nave, choir, or apse (aisles around the apse are usually called ambulatories). The aisle is often set off by columns or by an arcade.