Sundress

It is commonly a bodice-style sleeveless dress, typically with a wide neckline and thin shoulder straps, and may be backless.

While the word "sundress" was first used in the early 1940s, they really came into vogue in the 1950s and were especially popularized by Lilly Pulitzer in the 1960s.

[1] The sundress is considered by many people to be both more practical and more comfortable than other varieties of dresses, particularly in warm or hot weather.

They are typically but not always, sleeveless and collarless, with a wide neckline and thin shoulder straps.

Since the 1940s, a sundress-like one-piece swimsuit appeared,[4] has come into use, though some now find it matronly or gaudy.