A sundress is an informal or casual dress intended to be worn in warm weather
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Typically in a lightweight fabric, most commonly cotton, and usually loose-fitting. It is commonly a bodice style, sleeveless dress, typically with a wide neckline and thin shoulder straps, and maybe backless. A sundress is typically worn without a layering top and is not typically worn over a blouse, sweater or t-shirt, nor with leggings.
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