An aisle is, in general, a space for walking with rows of non-walking spaces on both sides. Aisles with seating on both sides can be seen in airplanes, certain types of buildings, such as churches, cathedrals, synagogues, meeting halls, parliaments and legislatures, courtrooms, theatres, and in certain types of passenger vehicles. Their floors may be flat or, as in theatres, stepped upwards from a stage.

Aisles can also be seen in shops, warehouses, and factories, where rather than seats, they have shelving to either side. In warehouses and factories, aisles may be defined by storage pallets, and in factories, aisles may separate work areas. In health clubs, exercise equipment is normally arranged in aisles.

Aisles are distinguished from corridors, hallways, walkways, footpaths, pavements (American English sidewalks), trails, paths and (enclosed) "open areas" by lying between other open spaces or areas of seating, but enclosed within a structure.