GALLERY (through Ital. galleria, from Med. Lat. galeria, of which the origin is unknown),  a covered passage or space outside a main wall, sometimes used as a verandah if on the ground floor, and as a balcony if on an upper floor and supported by columns, piers or corbels; similarly the upper seats in a theatre or a church, on either side as in many 17th-century churches, or across the west end under the organ. The word is also used of an internal passage primarily provided to place various rooms in communication with one another; but if of narrow width this is usually called a corridor or passage. When of sufficient width the gallery is utilized to exhibit pictures and other art treasures. In the 16th century the picture gallery formed the largest room or hall in English mansions, with wainscoted walls and a richly decorated plaster ceiling; the principal examples are those of Audley End, Essex (226 ft. by 34 ft.); Hardwick, Derbyshire (166 ft. by 22 ft.); Hatfield, Hertfordshire (163 ft. by 19 ft. 6 in.); Aston Hall, near Birmingham (136 ft. by 18 ft.); Haddon Hall, Derbyshire (116 ft. by 17 ft.); and Montacute in Somersetshire (189 ft. by 22 ft.). Hence the application of the term to art museums (the National Gallery, etc.) and also to smaller rooms with top-light in which temporary exhibitions are held.
 Du Cange, Glossarium, s.v. "Galeria," suggests an origin from galera, a galley, on the analogy of "nave," from navis, the galley being a long and narrow ship; but, he adds, alii alia opinantur.
Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)