Derived from the French for "wing," an aisle, historically, was an extension of a side or "wing" of the nave. It was built to enlarge the seating capacity of the church. This extension typically had a separate and lower roof. The aisle was separated from the central nave of the church by a passageway. The term was eventually applied to the passageways themselves, including the central passageway of the nave or "center aisle." Although the expression "center aisle" is technically a contradiction of terms, it is common usage in the church today.
Glossary definitions provided courtesy of Church Publishing Incorporated, New York, NY,(All Rights reserved) from "An Episcopal Dictionary of the Church, A User Friendly Reference for Episcopalians," Don S. Armentrout and Robert Boak Slocum, editors.