A gown (medieval Latin gunna) is a (usually) loose outer garment from knee- to full-length worn by men and women in Europe from the early Middle Ages to the seventeenth century (and continuing today in certain professions); later, gown was applied to any woman's garment consisting of a bodice and attached skirt.
A long, loosely-fitted gown called a Banyan was worn by men in the eighteenth century as an informal coat.
The gowns worn today by academics, judges, and some clergy derive directly from the everyday garments worn by their medieval predecessors, formalized into a uniform in the course of the sixteenth and seventeenth centuries.
Before the Victorian period, the word "dress" usually referred to a general overall mode of attire for either men or women (such as in the phrases "Evening Dress", "Morning Dress", "Travelling Dress", "Full Dress" etc.), rather than to any specific garment — and the most-used English word for a woman's skirted garment was "gown" (as in Jane Austen's novels).
By the early twentieth century, both gown and frock were essentially synonymous with dress, although gown was more often used for a formal or heavy garment and frock for a light-weight or informal one.
Only in the last few decades has gown lost its general meaning of a woman's garment in the US in favor of dress. Today the usage is chiefly British except in specialized, formal cases such as ball gown or in historical senses.
Arnold, Janet: Patterns of Fashion 2: Englishwomen's Dresses and Their Construction C.1860-1940, Wace 1966, Macmillan 1972. Revised metric edition, Drama Books 1977. ISBN 0-89676-027-8
Ashelford, Jane: The Art of Dress: Clothing and Society 1500-1914, Abrams, 1996. ISBN 0-8109-6317-5
Black, J. Anderson and Madge Garland: A History of Fashion, Morrow, 1975. ISBN 0-688-02893-4