What is the difference between a maxi dress and a sundress?

Maxy dress is compatable dress

Just a difference in wording, they are both muumuus.

A maxi dress is a floor or ankle length informal dress. Maxi dresses are formfitting at the top and loose flowing at the bottom, cut to flow over the body. ... A sundress is an informal or casual dress intended to be worn in warm weather, typically in a lightweight fabric, most commonly cotton, and usually loose-fitting.

A 'maxi' dress is full length, to the ankles.

A 'sundress' is a dress you wear when you live in the tropics - usually quite short, a light fabric, sleeveless and possibly cut low at the back, or with shoulder straps.

Maxi refers to hem length, sundress refers to style and fabric. A maxi dress is any day or casual dress with an ankle length hemline. It's not for formal occasions. It's for everything that is casual, elegant, things where shorts are too casual and an evening  dress too much. A sundress usually short sleeve or sleeveless, made from cool, comfortable fabrics and can have any length of hem from short to long and anything inbetween. A sundress is cool and comfortable on warm sunny days. Sundresses are casual and informal. 

one is long and the other is see through