The difference between a dress and a gown is largely one of semantics and personal preference. The word "gown" tends to refer to more formal garments, like those worn for weddings, proms or balls. A "dress" can be any one-piece garment with a skirt of any length, and can be either formal or informal. While many gowns are referred to as dresses, like those worn to a wedding or prom, people seldom hear less formal garments referred to this way. For example, a sundress is never a sungown.
All gowns are dresses, but not all dresses are gowns. In earlier English examples from the 16th century onward, the word "dress" referred to both men and women’s clothing ensembles. It might be modified by terms like evening, traveling, riding, and people can still see examples of this on invitations to certain events. An invitation might state that a function requires semi-formal dress or attire, for example. When an invitation states that the occasion is formal, women are expected to wear an evening gown.
When the term is used strictly, a gown is fancy and frivolous wear, with a tight fitting bodice and a full floor-length skirt. Certain close-fitting styles, like the form fitting types worn by actresses to award ceremonies, are still often considered to be gowns. Attending the Oscars, for example, tends to require formal eveningwear for both men and women, though notable exceptions have been fashion disasters of the past.
Another related term is frock. A frock may also be a less formal dress, but may also be an evening or formal gown. The word "frock" is more popular in the UK than in the US for referring to any one-piece garment with a skirt.
An exception to these general definitions is a graduation gown, which is worn over clothing. A woman might wear a dress under such a garment if she is planning to attend graduation celebrations after the ceremony. Usually, the outfit is flowing and oversized, and it is worn over other clothes, though a few graduates have tried to get away with wearing only the gown as part of a prank or bet. This may be cause for amusement but is considered inappropriate for the event of graduation from high school or college.