Gallery. In church architecture, an upper storey over an aisle, opening on to the nave. Also called a tribune and often, wrongly, a triforium. A false gallery is a middle storey comparable in height to a gallery but but opening into the roof space over the aisles without external windows. Found as an exterior feature with continuous small open arcading in medieval Italian and German churches, and sometims called a dwarf gallery.
In secular architecture: 1. A platform or mezzanine supported on columns or brackets and overlooking the main interior space of a building, e.g. a theater. 2. A long room, often on an upper floor, for recreation and entertainment, and sometimes used for the display of paintings, etc. (hence the modern term 'art gallery'). In England such rooms were often called 'long galleries'. The most famous if the Galerie des Glaces, Versailles (begun 1678), widely copied in palace architecture. 3. In Italy, a shopping arcade is called a galleria.
-- John Fleming, Hugh Honour & Nikolaus Pevsner, 1991
The Penguin Dictionary of Architecture
Penguin Books, London, England