An aisle is a passage on either sides of a nave in a basilican church. It is narrower than the nave and is usually separated from it by a row of columns or arches. The purpose of aisles is to enlarge the interior of a church beyond the nave.
The aisle roof is lower than that of the nave. In Syria, North Africa, and Europe it formed a lean-to, but in Egypt it was usually flat.
Dictionary of church terms. — St. GEORGE'S COPTIC ORTHODOX CHURCH SPORTING - ALEXANDRIA EGYPT. Fr. Tadros Yacoub Maiaty.