An aisle is, in church architecture, a passageway on either side of the nave. The aisles are separated from the nave by arcades or colonnades.

In most churches with aisles, they stop at the transept, but they may also continue through the transept and even around the choir. The different parts may be referred to as nave-aisles, transept-aisles or choir-aisles.

Aisles that continue around the choir may form an ambulatory.