Refers to religious or secular buildings characterized by an oblong plan divided into a nave with two or more side aisles, the former higher and wider than the latter and generally lit by clerestory windows; usually terminated by an apse. In Roman and Early Christian basilicas, the ground plan typically was a parallelogram in which the width was not greater than one-half of the length and not less than one-third of it. When there was more space in the length, porticoes were built on the short sides. The middle space was separated by columns from a lower ambulatory or portico; the width of the ambulatory equaled the height of the columns and measured one-third of the width of the central space. Vaulting could be used in the side aisles, but a flat roof was used for the very broad middle nave. Eventually basilicas were built with 5 and 7 aisles; a transept was often placed between the nave and the apse. The roofing of the transept together with the apse and portico produced variety in the exterior of basilicas.

Source: AAT

http://vocab.getty.edu/page/aat/300170443