We see "dress" and "gown" used interchangeably in historical romance novels thanks to our modern understanding of what the words mean, however, our modern use of the word "dress" is different from the Georgian era usage.

"Dress" was not used to reference the garment a woman wore. "Gown" was used instead, and that referred to anything from a morning gown to a ball gown, regardless how formal or informal. "Dress" referred to the overall type of attire worn by either gender, and that type of attire was classified and categorized by the time of day and occasion, such as "military dress" or "court dress," which is not a reference to the garment but the type of attire.

There were three main types of dress: full, half, and undress.

These types referred to the time of day or occasion rather than to how fully dressed someone was (so someone in their nightrail was not considered to be in "undress").

Full Dress meant evening attire, specifically opera, court, and ball. So for Full Dress at the ball, a gentleman would wear his ball dress (ie his three piece suit) and a lady her ball gown.

Half Dress meant afternoon attire, specifically opera and dinner.

Undress meant pretty much everything else, or morning attire, so morning dress, walking dress, carriage dress, promenade dress, afternoon dress, and riding habit. Again, don't get "dress" and "gown" confused. A lady would wear her morning gown for her morning dress while in undress. Not at all a head spinner, right?

Get to know this better with Jane Austen UK's post:

Undress, Half Dress, Full Dress: Making Sense of it All